CHAPTER XVI.

THE JURASSIC PERIOD.

Resting upon the Trias, with perfect conformity, and with an almost undeterminable junction, we have the great series of deposits which are known as the Oolitic Rocks, from the common occurrence in them of oolitic limestones, or as the Jurassic Rocks, from their being largely developed in the mountain-range of the Jura, on the western borders of Switzerland. Sediments of this series occupy extensive areas in Great Britain, on the continent of Europe, and in India. In North America, limestones and marls of this age have been detected in "the Black Hills, the Laramie range, and other eastern ridges of the Rocky Mountains; also over the Pacific slope, in the Uintah, Wahsatch, and Humboldt Mountains, and in the Sierra Nevada" (Dana); but in these regions their extent is still unknown, and their precise subdivisions have not been determined. Strata belonging to the Jurassic period are also known to occur in South America, in Australia, and in the Arctic zone. When fully developed, the Jurassic series is capable of subdivision into a number of minor groups, of which some are clearly distinguished by their mineral characters, whilst others are separated with equal certainty by the differences of the fossils that they contain. It will be sufficient for our present purpose, without entering into the more minute subdivisions of the series, to give here a very brief and general account of the main sub-groups of the Jurassic rocks, as developed in Britain—the arrangement of the Jura-formation of the continent of Europe agreeing in the main with that of England.

I. THE LIAS.—The base of the Jurassic series of Britain is formed by the great calcareo-argillaceous deposit of the "Lias," which usually rests conformably and almost inseparably upon the Rhaetic beds (the so-called "White Lias"), and passes up, generally conformably, into the calcareous sandstones of the Inferior Oolite. The Lias is divisible into the three principal groups of the Lower, Middle, and Upper Lias, as under, and these in turn contain many well-marked "zones;" so that the Lias has some claims to be considered as an independent formation, equivalent to all the remaining Oolitic rocks. The Lower Lias (Terrain Sinemurien of D'Orbigny) sometimes attains a thickness of as much as 600 feet, and consists of a great series of bluish or greyish laminated clays, alternating with thin bands of blue or grey limestone—the whole, when seen in quarries or cliffs from a little distance, assuming a characteristically striped and banded appearance. By means of particular species of Ammonites, taken along with other fossils which are confined to particular zones, the Lower Lias may be subdivided into several well-marked horizons. The Middle Lias, or Marlstone Series (Terrain Liasien of D'Orbigny), may reach a thickness of 200 feet, and consists of sands, arenaceous marls, and argillaceous limestones, sometimes with ferruginous beds. The Upper Lias (Terrain Toarcien of D'Orbigny) attains a thickness of 300 feet, and consists principally of shales below, passing upwards into arenaceous strata.

II. THE LOWER OOLITES.—Above the Lias comes a complex series of partly arenaceous and argillaceous, but principally calcareous strata, of which the following are the more important groups: a, The Inferior Oolite (Terrain Bajocien of D'Orbigny), consisting of more than 200 feet of oolitic limestones, sometimes more or less sandy; b, The Fuller's Earth, a series of shales, clays, and marls, about 120 feet in thickness; c, The Great Oolite or Bath Oolite (Terrain Bathonien of D'Orbigny), consisting principally of oolitic limestones, and attaining a thickness of about 130 feet. The well-known "Stonesfield Slates" belong to this horizon; and the locally developed "Bradford Clay," "Corn brash," and "Forest-marble" may be regarded as constituting the summit of this group.

III. THE MIDDLE OOLITES.—The central portion of the Jurassic series of Britain is formed by a great argillaceous deposit, capped by calcareous strata, as follows: a, The Oxford Clay (Terrain Callovien and Terrain Oxfordien of D'Orbigny), consisting of dark-coloured laminated clays, sometimes reaching a thickness of 700 feet, and in places having its lower portion developed into a hard calcareous sandstone ("Kelloway Rock"); b, The Coral-Rag (Terrain Corallien of D'Orbigny, "Nerinean Limestone" of the Jura, "Diceras Limestone" of the Alps), consisting, when typically developed, of a central mass of oolitic limestone, underlaid and surmounted by calcareous grits.

IV. THE UPPER OOLITES.—a, The base of the Upper Oolites of Britain is constituted by a great thickness (600 feet or more) of laminated, sometimes carbonaceous or bituminous clays, which are known as the Kimmeridge Clay (Terrain Kimmeridgien of D'Orbigny); b, The Portland Beds (Terrain Portlandien of D'Orbigny) succeed the Kimmeridge clay, and consist inferiorly of sandy beds surmounted by oolitic limestones ("Portland Stone"), the whole series attaining a thickness of 150 feet or more, and containing marine fossils; c, The Purbeck Beds are apparently peculiar to Great Britain, where they form the summit of the entire Oolitic series, attaining a total thickness of from 150 to 200 feet. The Purbeck beds consist of arenaceous, argillaceous, and calcareous strata, which can be shown by their fossils to consist of a most remarkable alternation of fresh-water, brackish-water, and purely marine sediments, together with old land-surfaces, or vegetable soils, which contain the upright stems of trees, and are locally known as "Dirt-beds."

One of the most important of the Jurassic deposits of the continent of Europe, which is believed to be on the horizon of the Coral-rag or of the lower part of the Upper Oolites, is the "Solenhofen Slate" of Bavaria, an exceedingly fine-grained limestone, which is largely used in lithography, and is celebrated for the number and beauty of its organic remains, and especially for those of Vertebrate animals.

The subjoined sketch-section (fig. 159) exhibits in a diagrammatic form the general succession of the Jurassic rocks of Britain.

Regarded as a whole, the Jurassic formation is essentially marine; and though remains of drifted plants, and of insects and other air-breathing animals, are not uncommon, the fossils of the formation are in the main marine. In the Purbeck series of Britain, anticipatory of the great river-deposit of the Wealden, there are fresh-water, brackish-water, and even terrestrial strata, indicating that the floor of the Oolitic ocean was undergoing upheaval, and that the marine conditions which had formerly prevailed were nearly at an end. In places also, as in Yorkshire and Sutherlandshire, are found actual beds of coal: but the great bulk of the formation is an indubitable sea-deposit; and its limestones, oolitic as they commonly are, nevertheless are composed largely of the comminuted skeletons of marine animals. Owing to the enormous number and variety of the organic remains which have been yielded by the richly fossiliferous strata of the Oolitic series, it will not be possible here to do more than to give an outline-sketch of the principal forms of life which characterise the Jurassic period as a whole. It is to be remembered, however, that every minor group of the Jurassic formation has its own peculiar fossils, and that by the labours of such eminent observers as Quenstedt, Oppel, D'Orbigny, Wright, De la Beche, Tate, and others, the entire series of Jurassic sediments admits of a more complete and more elaborate subdivision into zones characterised by special life-forms than has as yet been found practicable in the case of any other rock-series.



The plants of the Jurassic period consist principally of Ferns, Cycads, and Conifers—agreeing in this respect, therefore, with those of the preceding Triassic formation. The Ferns are very abundant, and belong partly to old and partly to new genera. The Cycads are also very abundant, and, on the whole, constitute the most marked feature of the Jurassic vegetation, many genera of this group being known (Pterophyllum, Otozamites, Zamites, Crossozamia, Williamsonia, Bucklandia, &c.) The so-called "dirt-bed" of the Purbeck series consists of an ancient soil, in which stand erect the trunks of Conifers and the silicified stools of Cycads of the genus Mantellia (fig.160). The Coniferoe of the Jurassic are represented by various forms more or less nearly allied to the existing Araucarioe; and these are known not only by their stems or branches, but also in some cases by their cones. We meet, also, with the remains of undoubted Endogenous plants, the most important of which are the fruits of forms allied to the existing Screw-pines (Pandaneoe), such as Podocarya and Kaidacarpum. So far, however, no remains of Palms have been found; nor are we acquainted with any Jurassic plants which could be certainly referred to the great "Angiospermous" group of the Exogens, including the majority of our ordinary plants and trees.

Amongst animals, the Protozoans are well represented in the Jurassic deposits by numerous Foraminifers and Sponges; as are the Coelenterates by numerous Corals. Remains of these last-mentioned organisms are extremely abundant in some of the limestones of the formation, such as the "Coral-rag" and the Great Oolite; and the former of these may fairly be considered as an ancient "reef." The Rugose Corals have not hitherto been detected in the Jurassic rocks; and the "Tabulate Corals," so-called, are represented only by examples of the modern genus Millepora. With this exception, all the Jurassic Corals belong to the great group which predominates in recent seas (Zoantharia sclerodermata); and the majority belong to the important reef-building family of the "Star-corals" (Astroeidoe). The form here figured (Thecosmilia annularis, fig. 161) is one of the characteristic species of the Coral-rag.



The Echinoderms are very numerous and abundant fossils in the Jurassic series, and are represented by Sea-lilies, Sea-urchins, Star-fishes, and Brittle-stars. The Crinoids are still common, and some of the limestones of the series are largely composed of the debris of these organisms. Most of the Jurassic forms resemble those with which we are already familiar, in having the body permanently attached to some foreign object by means of a longer or shorter jointed stalk or "column." One of the most characteristic Jurassic genera of these "stalked" Crinoids (though not exclusively confined to this period) is Pentacrinus (fig. 162). In this genus, the column is five-sided, with whorls of "side-arms;" and the arms are long, slender, and branched. The genus is represented at the present day by the beautiful "Medusa-head Pentacrinite" (Pentacrinus caput-medusoe). Another characteristic Oolitic genus is Apiocrinus, comprising the so-called "Pear Encrinites." In this group the column is long and rounded, with a dilated base, and having its uppermost joints expanded so as to form, with the cup itself, a pear-shaped mass, from the summit of which spring the comparatively short arms. Besides the "stalked" Crinoids, the Jurassic rocks have yielded the remains of the higher group of the "free" Crinoids, such as Saccosoma. These forms resemble the existing "Feather-stars" (Comatula) in being attached when young to some foreign body by means of a jointed stem, from which they detach themselves when fully grown to lead an independent existence. In this later stage of their life, therefore, they closely resemble the Brittle-stars in appearance. True Star-fishes (Asteroids) and Brittle-stars (Ophiuroids) are abundant in the Jurassic rocks, and the Sea-urchins (Echinoids) are so numerous and so well preserved as to constitute quite a marked feature of some beds of the series. All the Oolitic urchins agree with the modern Echinoids in having the shell composed of no more than twenty rows of plates. Many different genera are known, and a characteristic species of the Middle Oolites (Hemicidaris crenularis, fig. 163) is here figured.



Passing over the Annelides, which, though not uncommon, are of little special interest, we come to the Articulates, which also require little notice. Amongst the Crustaceans, whilst the little Water-fleas (Ostracoda) are still abundant, the most marked feature is the predominance which is now assumed by the Decapods—the highest of the known groups of the class. True Crabs (Brachyura) are by no means unknown; but the principal Oolitic Decapods belonged to the "Long-tailed" group (Macrura), of which the existing Lobsters, Prawns, and Shrimps are members. The fine-grained lithographic slates of Solenhofen are especially famous as a depot for the remains of these Crustaceans, and a characteristic species from this locality (Eryon arctiformis, fig. 164) is here represented. Amongst the air-breathing Articulates, we meet in the Oolitic rocks with the remains of Spiders (Arachnida), Centipedes (Myriapoda), and numerous true Insects (Insecta). In connection with the last-mentioned of these groups, it is of interest to note the occurrence of the oldest known fossil Butterfly—the Paloeontina Oolitica of the Stonesfield slate—the relationships of which appear to be with some of the living Butterflies of Tropical America.



Coming to the Mollusca, the Polyzoans, numerous and beautiful as they are, must be at once dismissed; but the Brachiopods deserve a moment's attention. The Jurassic Lamp-shells (fig. 165) do not fill by any means such a predominant place in the marine fauna of the period, as in many Palaeozoic deposits, but they are still individually numerous. The two ancient genera Leptoena (fig. 165, a) and Spirifera (fig. 165, b), dating the one from the Lower and the other from the Upper Silurian, appear here for the last time upon the scene, but they have not hitherto been recognised in deposits later than the Lias. The great majority of the Jurassic Brachiopods, however, belong to the genera Terebratula (fig. 165, c, e, f) and Rhynchonella (fig. 165. d), both of which are represented by living forms at the present day. The Terebratuloe, in particular, are very abundant, and the species are often confined to special horizons in the series.



Remains of Bivalves (Lamellibranchiata) are very numerous in the Jurassic deposits, and in many cases highly characteristic. In the marine beds of the Oolites, which constitute by far the greater portion of the whole formation, the Bivalyes are of course marine, and belong to such genera as Trigonia, Lima, Pholadomya, Cardinia, Avicula, Hippopodium, &c.; but in the Purbeck beds, at the summit of the series, we find bands of Oysters alternating with strata containing fresh-water or brackish-water Bivalves, such as Cyrenoe and Corbuloe. The predominant Bivalves of the Jurassic, however, are the Oysters, which occur under many forms, and often in vast numbers, particular species being commonly restricted to particular horizons. Thus of the true Oysters, Ostrea distorta is characteristic of the Purbeck series, where it forms a bed twelve feet in thickness, known locally as the "Cinder-bed;" Ostrea expansa abounds in the Portland beds; Ostrea deltoidea is characteristic of the Kimmeridge clay; Ostrea gregaria predominates in the Coral-rag; Ostrea acuminata characterises the small group of the Fuller's Earth; whilst the plaited Ostrea Marshii (fig. 166) is a common shell in the Lower and Middle Oolites. Besides the more typical Oysters, the Oolitic rocks abound in examples of the singularly unsymmetrical forms belonging to the genera Exogyra and Gryphoea (fig. 167). In the former of these are included Oysters with the beaks "reversed"—that is to say, turned towards the hinder part of the shell; whilst in the latter are Oysters in which the lower valve of the shell is much the largest, and has a large incurved beak, whilst the upper valve is small and concave. One of the most characteristic Exogyroe is the E. Virgula of the Oxford Clay, and of the same horizon on the Continent; and the Gryphoea incurva (fig. 167) is equally abundant in, and characteristic of, the formation of the Lias. Lastly, we may notice the extraordinary shells belonging to the genus Diceras (fig. 168), which are exclusively confined to the Middle Oolites. In this formation in the Alps they occur in such abundance as to give rise to the name of "Calcaire a Dicerates," applied to beds of the same age as the Coral-rag of Britain. The genus Diceras belongs to the same family as the "Thorny Clams" (Chama) of the present day—the shell being composed of nearly equally-sized valves, the beaks of which are extremely prominent and twisted into a spiral. The shell was attached to some foreign body by the beak of one of its valves.



Amongst the Jurassic Univalves (Gasteropoda) there are many examples of the ancient and long-lived Pleurotomaria; but on the whole the Univalves begin to have a modern aspect. The round-mouthed ("holostomatous"), vegetable-eating Sea-snails, such as the Limpets (Patellidoe), the Nerites (Nerita), the Turritelloe, Chemnitzioe, &c., still hold a predominant place. The two most noticeable genera of this group are Cerithium and Nerinoea—the former of these attaining great importance in the Tertiary and Recent seas, whilst the latter (fig. 169) is highly characteristic of the Jurassic series, though not exclusively confined to it. One of the limestones of the Jura, believed to be of the age of the Coral-rag (Middle Oolite) of Britain, abounds to such an extent in the turreted shells of Nerinoea as to have gained the name of "Calcaire a Nerinees." In addition to forms such as the preceding, we now for the first time meet, in any force, with the Carnivorous Univalves, in which the mouth of the shell is notched or produced into a canal, giving rise to the technical name of "siphonostomatous" applied to the shell. Some of the carnivorous forms belong to extinct types, such as the Purpuroidea of the Great Oolite; but others are referable to well-known existing genera. Thus we meet here with species of the familiar groups of the Whelks (Buccinum), the Spindle-shells (Fusus), the Spider-shells (Pteroceras), Murex, Rostellaria, and others which are not at present known to occur in any earlier formation.

Amongst the Wing-shells (Pteropoda), it is sufficient to mark the final appearance in the Lias of the ancient genus Conularia.



Lastly, the order of the Cephalopoda, in both its Tetrabranchiate and Dibranchiate sections, undergoes a vast development in the Jurassic period. The old and comparatively simple genus Nautilus is still well represented, one species being very similar to the living Pearly Nautilus (N. Pompilius); but the Orthocerata and Goniatites of the Trias have finally disappeared; and the great majority of the Tetrabranchiate forms are referable to the comprehensive genus Ammonites, with its many sub-genera and its hundreds of recorded species. The shell in Ammonites is in the form of a flat spiral, all the coils of which are in contact (figs. 170 and 171). The innermost whorls of the shell are more or less concealed; and the body-chamber is elongated and narrow, rather than expanded towards the mouth. The tube or siphuncle which runs through the air-chambers is placed on the dorsal or convex side of the shell; but the principal character which distinguishes Ammonites from Goniatites and Ceratites is the wonderfully complex manner in which the septa, or partitions between the air-chambers, are folded and undulated. To such an extent does this take place, that the edges of the septa, when exposed by the removal of the shell-substance, present in an exaggerated manner the appearance exhibited by an elaborately-dressed shirt-frill when viewed edgewise. The species of Ammonites range from the Carboniferous to the Chalk; but they have not been found in deposits older than the Secondary, in any region except India; and they are therefore to be regarded as essentially Mesozoic fossils. Within these limits, each formation is characterised by particular species, the number of individuals being often very great, and the size which is sometimes attained being nothing short of gigantic. In the Lias, particular species of Ammonites may succeed one another regularly, each having a more or less definite horizon, which it does not transgress. It is thus possible to distinguish a certain number of zones, each characterised by a particular Ammonite, together with other associated fossils. Some of these zones are very persistent and extend over very wide areas, thus affording valuable aid to the geologist in his determination of rocks. It is to be remembered, however, that there are other species which are not thus restricted in their vertical range, even in the same formations in which definite zones occur.

[Illustartion: Fig. 172.—Beloteuthis subcostata Jurassic (Lias).]

The Cuttle-fishes or Dibranchiate Cephalopods constitute a feature in the life of the Jurassic period little less conspicuous and striking than that afforded by the multitudinous and varied chambered shells of the Ammonitidoe. The remains by which these animals are recognised are necessarily less perfect, as a rule, than those of the latter, as no external shell is present (except in rare and more modern groups), and the internal skeleton is not necessarily calcareous. Nevertheless, we have an ample record of the Cuttle-fishes of the Jurassic period, in the shape of the fossilised jaws or beak, the ink-bag, and, most commonly of all, the horny or calcareous structure which is embedded in the soft tissues, and is variously known as the "pen" or "bone." The beaks of Cuttle-fishes, though not abundant, are sufficiently plentiful to have earned for themselves the general title of "Rhyncholites;" and in their form and function they resemble the horny, parrot-like beak of the existing Cephalopods. The ink-bag or leathery sac in which the Cuttle-fishes store up the black pigment with which they obscure the water when attacked, owes its preservation to the fact that the colouring-matter which it contains is finely-divided carbon, and therefore nearly indestructible except by heat. Many of these ink-bags have been found in the Lias; and the colouring-matter is sometimes so well preserved that it has been, as an experiment, employed in painting as a fossil "sepia." The "pens" of the Cuttle-fishes are not commonly preserved, owing to their horny consistence, but they are not unknown. The form here figured (Beloteuthis subcostata, fig. 172) belonged to an old type essentially similar to our modern Calamaries, the skeleton of which consists of a horny shaft and two lateral wings, somewhat like a feather in general shape. When, on the other hand, the internal skeleton is calcareous, then it is very easily preserved in a fossil condition; and the abundance of remains of this nature in the Secondary rocks, combined with their apparent total absence in Palaeozoic strata, is a strong presumption in favour of the view that the order of the Cuttle-fishes did not come into existence till the commencement of the Mesozoic period. The great majority of the skeletons of this kind which are found in the Jurassic rocks belong to the great extinct family of the "Belemnites" (Belemnitidoa), which, so far as known, is entirely confined to rocks of Secondary age. From its pointed, generally cylindro-conical form, the skeleton of the Belemnite is popularly known as a "thunderbolt". (fig. 173, C). In its perfect condition—in which it is, however, rarely obtainable—the skeleton consists of a chambered conical shell (the "phragmacone"), the partitions between the chambers of which are pierced by a marginal tube or "siphuncle." This conical shell—curiously similar in its structure to the external shell of the Nautilus—is extended forwards into a horny "pen," and is sunk in a corresponding conical pit (fig. 173, B), excavated in the substance of a nearly cylindrical fibrous body or "guard," which projects backwards for a longer or shorter distance, and is the part most usually found in a fossil condition. Many different kinds of Belemnites are known, and their guards literally swarm in many parts of the Jurassic series, whilst some specimens attain very considerable dimensions. Not only is the internal skeleton known, but specimens of Belemnites and the nearly allied Belemnoteuthis have been found in some of the fine-grained sediments of the Jurassic formation, from which much has been learnt even as to the anatomy of the soft parts of the animal. Thus we know that the Belemnites were in many respects comparable with the existing Calamaries or Squids, the body being furnished with lateral fins, and the head carrying a circle of ten "arms," two of which were longer than the others (fig. 173, A). The suckers on the arms were provided, further, with horny hooks; there was a large ink-sac; and the mouth was armed with horny mandibles resembling in shape the beak of a parrot.



Coming next to the Vertebrates, we find that the Jurassic Fishes are still represented by Ganoids and Placoids. The Ganoids, however, unlike the old forms, now for the most part possess nearly or quite symmetrical ("homocercal") tails. A characteristic genus is Tetragonolepis (fig. 174), with its deep compressed body, its rhomboidal, closely-fitting scales, and its single long dorsal fin. Amongst the Placoids the teeth of true Sharks (Notidanus) occur for the first time; but by far the greater number of remains referable to this group are still the fin-spines and teeth of "Cestracionts," resembling the living Port-Jackson Shark. Some of these teeth are pointed (Hybodus); but others are rounded, and are adapted for crushing shell-fish. Of these latter, the commonest are the teeth of Acrodus (fig. 175), of which the hinder ones are of an elongated form, with a rounded surface, covered with fine transverse striae proceeding from a central longitudinal line. From their general form and striation, and their dark colour, these teeth are commonly called "fossil leeches" by the quarrymen.



The Amphibian group of the Labyrinthodonts, which was so extensively developed in the Trias, appears to have become extinct, no representative of the order having hitherto been detected in rocks of Jurassic age.



Much more important than the Fishes of the Jurassic series are the Reptiles, which are both very numerous, and belong to a great variety of types, some of these being very extraordinary in their anatomical structure. The predominant group is that of the "Enaliosaurs" or "Sea-lizards," divided into two great orders, represented respectively by the Ichthyosaurus and the Plesiosaurus.

The Ichthyosauri or "Fish-Lizards" are exclusively Mesozoic in their distribution, ranging from the Lias to the Chalk, but abounding especially in the former. They were huge Reptiles, of a fish-like form, with a hardly conspicuous neck (fig. 176), and probably possessing a simply smooth or wrinkled skin, since no traces of scales or bony integumentary plates have ever been discovered. The tail was long, and was probably furnished at its extremity with a powerful expansion of the skin, constituting a tail-fin similar to that possessed by the Whales. The limbs are also like those of Whales in the essentials of their structure, and in their being adapted to act as swimming-paddles. Unlike the Whales, however, the Ichthyosaurs possessed the hind-limbs as well as the fore-limbs, both pairs having the bones flattened out and the fingers completely enclosed in the skin, the arm and leg being at the same time greatly shortened. The limbs are thus converted into efficient "flippers," adapting the animal for an active existence in the sea. The different joints of the backbone (vertebrae) also show the same adaptation to an aquatic mode of life, being hollowed out at both ends, like the biconcave vertebrae of Fishes. The spinal column in this way was endowed with the flexibility necessary for an animal intended to pass the greater part of its time in water. Though the Ichthyosaurs are undoubtedly marine animals, there is, however, reason to believe that they occasionally came on shore, as they possess a strong bony arch, supporting the fore-limbs, such as would permit of partial, if laborious, terrestrial progression. The head is of enormous size, with greatly prolonged jaws, holding numerous powerful conical teeth lodged in a common groove. The nature of the dental apparatus is such as to leave no doubt as to the rapacious and predatory habits of the Ichthyosaurs—an inference which is further borne out by the examination of their petrified droppings, which are known to geologists as "coprolites," and which contain numerous fragments of the bones and scales of the Ganoid fishes which inhabited the same seas. The orbits are of huge size; and as the eyeball was protected, like that of birds, by a ring of bony plates in its outer coat, we even know that the pupils of the eyes were of correspondingly large dimensions. As these bony plates have the function of protecting the eye from injury under sudden changes of pressure in the surrounding medium, it has been inferred, with great probability, that the Ichthyosaurs were in the habit of diving to considerable depths in the sea. Some of the larger specimens of Ichthyosaurus which have been discovered in the Lias indicate an animal of from 20 to nearly 40 feet in length; and many species are known to have existed, whilst fragmentary remains of their skeletons are very abundant in some localities. We may therefore safely conclude that these colossal Reptiles were amongst the most formidable of the many tyrants of the Jurassic seas.



The Plesiosaurus (fig. 177) is another famous Oolitic Reptile, and, like the preceding, must have lived mainly or exclusively in the sea. It agrees with the Ichthyosaur in some important features of its organisation, especially in the fact that both pairs of limbs are converted into "flippers" or swimming-paddles, whilst the skin seems to have been equally destitute of any scaly or bony investiture. Unlike the Ichthyosaur, however, the Plesiosaur had the paddles placed far back, the tail being extremely short, and the neck greatly lengthened out, and composed of from twenty to forty vertebrae. The bodies of the vertebrae, also, are not deeply biconcave, but are flat, or only slightly cupped. The head is of relatively small size, with smaller orbits than those of the Ichthyosaur, and with a snout less elongated. The jaws, however, were armed with numerous conical teeth, inserted in distinct sockets. As regards the habits of the Plesiosaur, Dr Conybeare arrives at the following conclusions: "That it was aquatic is evident from the form of its paddles; that it was marine is almost equally so from the remains with which it is universally associated; that it may have occasionally visited the shore, the resemblance of its extremities to those of the Turtles may lead us to conjecture: its movements, however, must have been very awkward on land; and its long neck must have impeded its progress through the water, presenting a strong contrast to the organisation which so admirably fits the Ichthyosaurus to cut through the waves." As its respiratory organs were such that it must of necessity have required to obtain air frequently, we may conclude "that it swam upon or near the surface, arching back its long neck like a swan, and occasionally darting it down at the fish which happened to float within its reach. It may perhaps have lurked in shoal water along the coast, concealed amongst the sea-weed; and raising its nostrils to a level with the surface from a considerable depth, may have found a secure retreat from the assaults of powerful enemies; while the length and flexibility of its neck may have compensated for the want of strength in its jaws, and its incapacity for swift-motion through the water."

About twenty species of Plesiosaurus are known, ranging from the Lias to the Chalk, and specimens have been found indicating a length of from eighteen to twenty feet. The nearly related "Pliosaurs," however, with their huge heads and short necks, must have occasionally reached a length of at least forty feet—the skull in some species being eight, and the paddles six or seven feet long, whilst the teeth are a foot in length.



Another extraordinary group of Jurassic Reptiles is that of the "Winged Lizards" or Pterosauria. These are often spoken of collectively as "Pterodactyles," from Pterodactylus, the type-genus of the group. As now restricted, however, the genus Pterodactylus is more Cretaceous than Jurassic, and it is associated in the Oolitic rocks with the closely allied genera Dimorphodon and Rhamphorhynchus. In all three of these genera we have the same general structural organisation, involving a marvellous combination of characters, which we are in the habit of regarding as peculiar to Birds on the one hand, to Reptiles on another hand, and to the Flying Mammals or Bats in a third direction. The "Pterosaurs" are "Flying" Reptiles, in the true sense of the term, since they were indubitably possessed of the power of active locomotion in the air, after the manner of Birds. The so-called "Flying" Reptiles of the present day, such as the little Draco volans of the East Indies and Indian Archipelago, possess, on the other hand, no power of genuine flight, being merely able to sustain themselves in the air through the extensive leaps which they take from tree to tree, the wing-like expansions of the skin simply exercising the mechanical function of a parachute. The apparatus of flight in the "Pterosaurs" is of the most remarkable character, and most resembles the "wing" of a Bat, though very different in some important particulars. The "wing" of the Pterosaurs is like that of Bats, namely, in consisting of a thin leathery expansion of the skin which is attached to the sides of the body, and stretches between the fore and hind limbs, being mainly supported by an enormous elongation of certain of the digits of the hand. In the Bats, it is the four outer fingers which are thus lengthened out; but in the Pterosaurs, the wing-membrane is borne by a single immensely-extended finger (fig. 178). No trace of the actual wing-membrane itself has, of course, been found fossilised; but we could determine that the "Pterodactyles" possessed the power of flight, quite apart from the extraordinary conformation of the hand. The proofs of this are to be found partly in the fact that the breast-bone was furnished with an elevated ridge or keel, serving for the attachment of the great muscles of flight, and still more in the fact that the bones were hollow and were filled with air—a peculiarity wholly confined amongst living animals to Birds only. The skull of the Pterosaurs is long, light, and singularly bird-like in appearance—a resemblance which is further increased by the comparative length of the neck and the size of the vertebrae of this region (fig. 178). The jaws, however, unlike those of any existing Bird, were, with one exception to be noticed hereafter, furnished with conical teeth sunk in distinct sockets; and there was always a longer or shorter tail composed of distinct vertebrae; whereas in all existing Birds the tail is abbreviated, and the terminal vertebrae are amalgamated to form a single bone, which generally supports the great feathers of the tail.

Modern naturalists have been pretty generally agreed that the Pterosaurs should be regarded as a peculiar group of the Reptiles; though they have been and are still regarded by high authorities, like Professor Seeley, as being really referable to the Birds, or as forming a class by themselves. The chief points which separate them from Birds, as a class, are the character of the apparatus of flight, the entirely different structure of the fore-limb, the absence of feathers, the composition of the tail out of distinct vertebrae, and the general presence of conical teeth sunk in distinct sockets in the jaws. The gap between the Pterosaurs and the Birds has, however, been greatly lessened of late by the discovery of fossil animals (Ichthyornis and Hesperornis) with the skeleton proper to Birds combined with the presence of teeth in the jaws, and by the still more recent discovery of other fossil animals (Pteranodon) with a Pterosaurian skeleton, but without teeth; whilst the undoubtedly feathered Archoeopteryx possessed a long tail composed of separate vertebrae. Upon the whole, therefore, the relationships of the Pterosaurs cannot be regarded as absolutely settled. It seems certain, however, that they did not possess feathers—this implying that they were cold-blooded animals; and their affinities with Reptiles in this, as in other characters, are too strong to be overlooked.



The Pterosaurs are wholly Mesozoic, ranging from the Lias to the Chalk inclusive; and the fine-grained Lithographic Slate of Solenhofen has proved to be singularly rich in their remains. The genus Pterodactylus itself has the jaws toothed to the extremities with equal-sized conical teeth, and its species range from the Middle Oolites to the Cretaceous series, in connection with which they will be again noticed, together with the toothless genus Pteranodon. The genus Dimorphodon is Liassic, and is characterised by having the front teeth long and pointed, whilst the hinder teeth are small and lancet-shaped. Lastly, the singular genus Rhamphorhynchus, also from the Lower Oolites, is distinguished by the fact that there are teeth present in the hinder portions of both jaws; but the front portions are toothless, and may have constituted a horny beak. Like most of the other Jurassic Pterosaurs, Rhamphorhynchus (fig. 179) does not seem to have been much bigger than a pigeon, in this respect falling far below the giant "Dragons" of the Cretaceous period. It differed from its relatives, not only in the armature of the mouth, but also in the fact that the tail was of considerable length. With regard to its habits and mode of life, Professor Phillips remarks that, "gifted with ample means of flight, able at least to perch on rocks and scuffle along the shore, perhaps competent to dive, though not so well as a Palmiped bird, many fishes must have yielded to the cruel beak and sharp teeth of Rhamphorhynchus. If we ask to which of the many families of Birds the analogy of structure and probable way of life would lead us to assimilate Rhamphorhynchus, the answer must point to the swimming races with long wings, clawed feet, hooked beak, and habits or violence and voracity; and for preference, the shortness of the legs, and other circumstances, may be held to claim for the Stonesfield fossil a more than fanciful similitude to the groups of Cormorants, and other marine divers, which constitute an effective part of the picturesque army of robbers of the sea."

Another extraordinary and interesting group of the Mesozoic Reptiles is constituted by the Deinosauria, comprising a series of mostly gigantic forms, which range from the Trias to the Chalk. All the "Deinosaurs" are possessed of the two pairs of limbs proper to Vertebrate animals, and these organs are in the main adapted for walking on the dry land. Thus, whilst the Mesozoic seas swarmed with the huge Ichthyosaurs and Plesiosaurs, and whilst the air was tenanted by the Dragon-like Pterosaurs, the land-surfaces of the Secondary period were peopled by numerous forms of Deinosaurs, some of them of even more gigantic dimensions than their marine brethren. The limbs of the Deinosaurs are, as just said, adapted for progression on the land; but in some cases, at any rate, the hind-limbs were much longer and stronger than the fore-limbs; and there seems to be no reason to doubt that many of these forms possessed the power of walking, temporarily or permanently, on their hind-legs, thus presenting a singular resemblance to Birds. Some very curious and striking points connected with the structure of the skeleton have also been shown to connect these strange Reptiles with the true Birds; and such high authorities as Professors Huxley and Cope are of opinion that the Deinosaurs are distinctly related to this class, being in some respects intermediate between the proper Reptiles and the great wingless Birds, like the Ostrich and Cassowary. On the other hand, Professor Owen has shown that the Deinosaurs possess some weighty points of relationship with the so-called "Pachydermatous" Quadrupeds, such as the Rhinoceros and Hippopotamus. The most important Jurassic genera of Deinosauria are Megalosaurus and Cetiosaurus, both of which extend their range into the Cretaceous period, in which flourished, as we shall see, some other well-known members of this order.



Megalosaurus attained gigantic dimensions, its thigh and shank bones measuring each about three feet in length, and its total length, including the tail, being estimated at from forty to fifty feet. As the head of the thigh-bone is set on nearly at right angles with the shaft, whilst all the long bones of the skeleton are hollowed out internally for the reception of the marrow, there can be no doubt as to the terrestrial habits of the animal. The skull (fig. 180) was of large size, four or five feet in length, and the jaws were armed with a series of powerful pointed teeth. The teeth are conical in shape, but are strongly compressed towards their summits, their lateral edges being finely serrated. In their form and their saw-like edges, they resemble the teeth of the "Sabre-toothed Tiger" (Machairodus), and they render it certain that the Megalosaur was in the highest degree destructive and carnivorous in its habits. So far as is known, the skin was not furnished with any armour of scales or bony plates; and the fore-limbs are so disproportionately small as compared with the hind-limbs, that this huge Reptile—like the equally huge Iguanodon—may be conjectured to have commonly supported itself on its hind-legs only.

The Cetiosaur attained dimensions even greater than those of the Megalosaur, one of the largest thigh-bones measuring over five feet in length and a foot in diameter in the middle, and the total length of the animal being probably not less than fifty feet. It was originally regarded as a gigantic Crocodile, but it has been shown to be a true Deinosaur. Having obtained a magnificent series of remains of this reptile, Professor Phillips has been able to determine many very interesting points as to the anatomy and habits of this colossal animal, the total length of which he estimates as being probably not less than sixty or seventy feet. As to its mode of life, this accomplished writer remarks:—

"Probably when 'standing at ease' not less than ten feet in height, and of a bulk in proportion, this creature was unmatched in magnitude and physical strength by any of the largest inhabitants of the Mesozoic land or sea. Did it live in the sea, in fresh waters, or on the land? This question cannot be answered, as in the case of Ichthyosaurus, by appeal to the accompanying organic remains; for some of the bones lie in marine deposits, others in situations marked by estuarine conditions, and, out of the Oxfordshire district, in Sussex, in fluviatile accumulations. Was it fitted to live exclusively in water? Such an idea was at one time entertained, in consequence of the biconcave character of the caudal vertebrae, and it is often suggested by the mere magnitude of the creature, which would seem to have an easier life while floating in water, than when painfully lifting its huge bulk, and moving with slow steps along the ground. But neither of these arguments is valid. The ancient earth was trodden by larger quadrupeds than our elephant; and the biconcave character of vertebrae, which is not uniform along the column in Cetiosaurus, is perhaps as much a character of a geological period as of a mechanical function of life. Good evidence of continual life in water is yielded in the case of Ichthyosaurus and other Enaliosaurs, by the articulating surfaces of their limb-bones, for these, all of them, to the last phalanx, have that slight and indefinite adjustment of the bones, with much intervening cartilage, which fits the leg to be both a flexible and forcible instrument of natation, much superior to the ordinary oar-blade of the boatman. On the contrary, in Cetiosaur, as well as in Megalosaur and Iguanodon, all the articulations are definite, and made so as to correspond to determinate movements in particular directions, and these are such as to be suited for walking. In particular, the femur, by its head projecting freely from the acetabulum, seems to claim a movement of free stepping more parallel to the line of the body, and more approaching to the vertical than the sprawling gait of the crocodile. The large claws concur in this indication of terrestrial habits. But, on the other hand, these characters are not contrary to the belief that the animal may have been amphibious; and the great vertical height of the anterior part of the tail seems to support this explanation, but it does not go further.... We have therefore a marsh-loving or river-side animal, dwelling amidst filicine, cycadaceous, and coniferous shrubs and trees full of insects and small mammalia. What was its usual diet? If ex ungue leonem, surely ex dente cibum. We have indeed but one tooth, and that small and incomplete. It resembles more the tooth of Iguanodon than that of any other reptile; for this reason it seems probable that the animal was nourished by similar vegetable food which abounded in the vicinity, and was not obliged to contend with Megalosaurus for a scanty supply of more stimulating diet."

All the groups of Jurassic Reptiles which we have hitherto been considering are wholly unrepresented at the present day, and do not even pass upwards into the Tertiary period. It may be mentioned, however, that the Oolitic deposits have also yielded the remains of Reptiles belonging to three of the existing orders of the class-namely, the Lizards (Lacertilia), the Turtles (Chelonia), and the Crocodiles (Crocodilia). The Lizards occur both in the marine strata of the Middle Oolites and also in the fresh-water beds of the Purbeck series; and they are of such a nature that their affinities with the typical Lacertilians of the present day cannot be disputed. The Chelonians, up to this point only known by the doubtful evidence of footprints in the Permian and Triassic sandstones, are here represented by unquestionable remains, indicating the existence of marine Turtles (the Chelone planiceps of the Portland Stone). No remains of Serpents (Ophidians) have as yet been detected in the Jurassic; but strata of this age have yielded the remains of numerous Crocodilians, which probably inhabited the sea. The most important member of this group is Teleosaurus, which attained a length of over thirty feet, and is in some respects allied to the living Gavials of India.



The great class of the Birds, as we have seen, is represented in rocks earlier than the Oolites simply by the not absolutely certain evidence of the three-toed footprints of the Connecticut Trias. In the Lithographic Slate of Solenhofen (Middle Oolite), there has been discovered, however, the at present unique skeleton of a Bird well known under the name of the Archoeopteryx macrura (figs. 181, 182). The only known specimen—now in the British Museum—unfortunately does not exhibit the skull; but the fine-grained matrix has preserved a number of the other bones of the skeleton, along with the impressions of the tail and wing feathers. From these remains we know that Archoeopteryx differed in some remarkable peculiarities of its structure from all existing members of the class of Birds. This extraordinary Bird (fig. 182) appears to have been about as big as a Rook—the tail being long and extremely slender, and composed of separate vertebrae, each of which supports a single pair of quill-feathers. In the flying Birds of the present day, as before mentioned, the terminal vertebrae of the tail are amalgamated to form a single bone ("ploughshare-bone"), which supports a cluster of tail-feathers; and the tail itself is short. In the embryos of existing Birds the tail is long, and is made up of separate vertebrae, and the same character is observed in many existing Reptiles. The tail of Archoeopteryx, therefore, is to be regarded as the permanent retention of an embryonic type of structure, or as an approximation to the characters of the Reptiles. Another remarkable point in connection with Archoeopteryx, in which it differs from all known Birds, is, that the wing was furnished with two free claws. From the presence of feathers, Archoeopteryx may be inferred to have been hot-blooded; and this character, taken along with the structure of the skeleton of the wing, may be held as sufficient to justify its being considered as belonging to the class of Birds. In the structure of the tail, however, it is singularly Reptilian; and there is reason to believe that its jaws were furnished with teeth sunk in distinct sockets, as is the case in no existing Bird. This conclusion, at any rate, is rendered highly probable by the recent discovery of "Toothed Birds" (Odonturnithes) in the Cretaceous rocks of North America.



The Mammals of the Jurassic period are known to us by a number of small forms which occur in the "Stonesfield Slate" (Great Oolite) and in the Purbeck beds (Upper Oolite). The remains of these are almost exclusively separated halves of the lower jaw, and they indicate the existence during the Oolitic period in Europe of a number of small "Pouched animals" (Marsupials). In the horizon of the Stonesfield Slate four genera of these little Quadrupeds have been described—viz., Amphilestes, Amphitherium, Phascolotherium, and Stereognathus. In Amphitherium (fig. 183), the molar teeth are furnished with small pointed eminences or "cusps;" and the animal was doubtless insectivorous. By Professor Owen, the highest living authority on the subject, Amphitherium is believed to be a small Marsupial, most nearly allied to the living Banded Ant-eater (Myrmecobius) of Australia (fig. 158). Amphilestes and Phascolotherium (fig. 184) are also believed by the same distinguished anatomist and palaeontologist to have been insect-eating Marsupials, and the latter is supposed to find its nearest living ally in the Opossums (Didelphys) of America. Lastly, the Stereognathus of the Stonesfield Slate is in a dubious position. It may have been a Marsupial; but, upon the whole, Professor Owen is inclined to believe that it must have been a hoofed and herbivorous Quadruped belonging to the series of the higher Mammals (Placentalia). In the Middle Purbeck beds, near to the close of the Oolitic period, we have also evidence of the existence of a number of small Mammals, all of which are probably Marsupials. Fourteen species are known, all of small size, the largest being no bigger than a Polecat or Hedgehog. The genera to which these little quadrupeds have been referred are Plagiaulax, Spalacotherium, Triconodon, and Galestes. The first of these (fig. 184, 4) is believed by Professor Owen to have been carnivorous in its habits; but other authorities maintain that it was most nearly allied to the living Kangaroo-rats (Hypsiprymnus) of Australia, and that it was essentially herbivorous. The remaining three genera appear to have been certainly insectivorous, and find their nearest living representatives in the Australian Phalangers and the American Opossums.

Finally, it is interesting to notice in how many respects the Jurassic fauna of Western Europe approached to that now inhabiting Australia. At the present day, Australia is almost wholly tenanted by Marsupials; upon its land-surface flourish Araucarioe and Cycadaceous plants, and in its seas swims the Port-Jackson Shark (Cestracion Philippi); whilst the Molluscan genus Trigonia is nowadays exclusively confined to the Australian coasts. In England, at the time of the deposition of the Jurassic rocks, we must have had a fauna and flora very closely resembling what we now see in Australia. The small Marsupials, Amphitherium, Phascolotherium, and others, prove that the Mammals were the same in order; cones of Araucarian pines, with tree-ferns and fronds of Cycads, occur throughout the Oolitic series; spine-bearing fishes, like the Port-Jackson Shark, are abundantly represented by genera such as Acrodus and Strophodus; and lastly, the genus Trigonia, now exclusively Australian, is represented in the Oolites by species which differ little from those now existing. Moreover, the discovery during recent years of the singular Mud-fish, the Ceratodus Fosteri in the rivers of Queensland, has added another and a very striking point of resemblance to those already mentioned; since this genus of Fishes, though preeminently Triassic, nevertheless extended its range into the Jurassic. Upon the whole, therefore, there is reason to conclude that Australia has undergone since the close of the Jurassic period fewer changes and vicissitudes than any other known region of the globe; and that this wonderful continent has therefore succeeded in preserving a greater number of the characteristic life-features of the Oolites than any other country with which we are acquainted.

LITERATURE.

The following list comprises some of the more important sources of information as to the rocks and fossils of the Jurassic series:—

(1) 'Geology of Oxford and the Thames Valley.' Phillips. (2) 'Geology of Yorkshire,' vol. ii. Phillips. (3) 'Memoirs of the Geological Survey of Great Britain.' (4) 'Geology of Cheltenham.' Murchison, 2d ed. Buckman. (5) 'Introduction to the Monograph of the Oolitic Asteriadae' (Palaeontographical Society). Wright. (6) "Zone of Avicula contorta and the Lower Lias of the South of England"—'Quart. Journ. Geol. Soc.,' vol. xvi., 1860. Wright. (7) "Oolites of Northamptonshire"—'Quart. Journ. Geol. Soc.,' vols. Xxvi. and xxix. Sharp. (8) 'Manual of Geology.' Dana. (9) 'Der Jura.' Quenstedt. (10) 'Das Floetzgebirge Wuerttembergs.' Quenstedt. (11) 'Jura Formation.' Oppel. (12) 'Paleontologie du Departement de la Moselle.' Terquem. (13) 'Cours elementaire de Paleontologie.' D'Orbigny. (14) 'Paleontologie Francaise.' D'Orbigny. (15) 'Fossil Echinodermata of the Oolitic Formation' (Palaeontographical Society). Wright. (16) 'Brachiopoda of the Oolitic Formation' (Palaeontographical Society). Davidson. (17) 'Mollusca of the Great Oolite' (Palaeontographical Society). Morris and Lycett. (18) 'Monograph of the Fossil Trigoniae' (Palaeontographical Society). Lycett. (19) 'Corals of the Oolitic Formation' (Palaeontographical Society). Edwards and Haime. (20) 'Supplement to the Corals of the Oolitic Formation' (Palaeontographical Society). Martin Duncan. (21) 'Monograph of the Belemnitidae' (Palaeontographical Society). Phillips. (22) 'Structure of the Belemnitidae' (Mem. Geol. Survey). Huxley. (23) 'Sur les Belemnites.' Blainville. (24) 'Cephalopoden.' Quenstedt. (25) 'Mineral Conchology.' Sowerby. (26) 'Jurassic Cephalopoda' (Palaeontologica Indica). Waagen. (27) 'Manual of the Mollusca.' Woodward. (28) 'Petrefaktenkunde.' Schlotheim. (29) 'Bridgewater Treatise.' Buckland. (30) 'Versteinerungen des Oolithengebirges.' Roemer. (31) 'Catalogue of British Fossils.' Morris. (32) 'Catalogue of Fossils in the Museum of Practical Geology.' Etheridge. (33) 'Beitraege zur Petrefaktenkunde.' Muenster. (34) 'Petrefacta Germaniae.' Goldfuss. (35) 'Lethaea Rossica.' Eichwald. (36) 'Fossil Fishes' (Decades of the Geol. Survey). Sir Philip Egerton. (37) 'Manual of Palaeontology.' Owen. (38) 'British Fossil Mammals and Birds.' Owen. (39) 'Monographs of the Fossil Reptiles of the Oolitic Formation' (Palaeontographical Society). Owen. (40) 'Fossil Mammals of the Mesozoic Formations' (Palaeontographical Society). Owen. (41) 'Catalogue of Ornithosauria.' Seeley. (42) "Classification of the Deinosauria"—'Quart. Journ. Geol. Soc.,' vol. xxvi., 1870. Huxley.



CHAPTER XVII.

THE CRETACEOUS PERIOD.

The next series of rocks in ascending order is the great and important series of the Cretaceous Rocks, so called from the general occurrence in the system of chalk (Lat. creta, chalk). As developed in Britain and Europe generally, the following leading subdivisions may be recognised in the Cretaceous series:—

1. Wealden, Lower Cretaceous. 2. Lower Greensand or Neocomian, / 3. Gault, 4. Upper Greensand, Upper Cretaceous. 5. Chalk, 6. Maestricht beds, /

I. Wealden.—The Wealden formation, though of considerable importance, is a local group, and is confined to the southeast of England, France, and some other parts of Europe. Its name is derived from the Weald, a district comprising parts of Surrey, Sussex, and Kent, where it is largely developed. Its lower portion, for a thickness of from 500 to 1000 feet, is arenaceous, and is known as the Hastings Sands. Its Upper portion, for a thickness of 150 to nearly 300 feet, is chiefly argillaceous, consisting of clays with sandy layers, and occasionally courses of limestone. The geological importance of the Wealden formation is very great, as it is undoubtedly the delta of an ancient river, being composed almost wholly of fresh-water beds, with a few brackish-water and even marine strata, intercalated in the lower portion. Its geographical extent, though uncertain, owing to the enormous denudation to which it has been subjected, is nevertheless great, since it extends from Dorsetshire to France, and occurs also in North Germany. Still, even if it were continuous between all these points, it would not be larger than the delta of such a modern river as the Ganges. The river which produced the Wealden series must have flowed from an ancient continent occupying what is now the Atlantic Ocean; and the time occupied in the formation of the Wealden must have been very great, though we have, of course, no data by which we can accurately calculate its duration.

The fossils of the Wealden series are, naturally, mostly the remains of such animals as we know at the present day as inhabiting rivers. We have, namely, fresh-water Mussels (Unio), River-snails (Paludina), and other fresh-water shells, with numerous little bivalved Crustaceans, and some fishes.

II. Lower Greensand (Neocomien of D'Orbigny).—The Wealden beds pass upward, often by insensible gradations, into the Lower Greensand. The name Lower Greensand is not an appropriate one, for green sands only occur sparingly and occasionally, and are found in other formations. For this reason it has been proposed to substitute for Lower Greensand the name Neocomian, derived from the town of Neufchatel—anciently called Neocomum—in Switzerland. If this name were adopted, as it ought to be, the Wealden beds would be called the Lower Neocomian.

The Lower Greensand or Neocomian of Britain has a thickness of about 850 feet, and consists of alternations of sands, sandstones, and clays, with occasional calcareous bands. The general colour of the series is dark brown, sometimes red; and the sands are occasionally green, from the presence of silicate of iron.

The fossils of the Lower Greensand are purely marine, and among the most characteristic are the shells of Cephalopods.

The most remarkable point, however, about the fossils of the Lower Cretaceous series, is their marked divergence from the fossils of the Upper Cretaceous rocks. Of 280 species of fossils in the Lower Cretaceous series, only 51, or about 18 per cent, pass on into the Upper Cretaceous. This break in the life of the two periods is accompanied by a decided physical break as well; for the Gault is often, if not always, unconformably superimposed on the Lower Greensand. At the same time, the Lower and Upper Cretaceous groups form a closely-connected and inseparable series, as shown by a comparison of their fossils with those of the underlying Jurassic rocks and the overlying Tertiary beds. Thus, in Britain no marine fossil is known to be common to the marine beds of the Upper Oolites and the Lower Greensand; and of more than 500 species of fossils in the Upper Cretaceous rocks, almost everyone died out before the formation of the lowest Tertiary strata, the only survivors being one Brachiopod and a few Foraminifera.

III. Gault (Aptien of D'Orbigny).—The lowest member of the Upper Cretaceous series is a stiff, dark-grey, blue, or brown clay, often worked for brick-making, and known as the Gault, from a provincial English term. It occurs chiefly in the south-east of England, but can be traced through France to the flanks of the Alps and Bavaria. It never exceeds 100 feet in thickness; but it contains many fossils, usually in a state of beautiful preservation.

IV. Upper Greensand (Albien of D'Orbigny; Unterquader and Lower Plaenerkalk of Germany).—The Gault is succeeded upward by the Upper Greensand, which varies in thickness from 3 up to 100 feet, and which derives its name from the occasional occurrence in it of green sands. These, however, are local and sometimes wanting, and the name "Upper Greensand" is to be regarded as a name and not a description. The group consists, in Britain, of sands and clays, sometimes with bands of calcareous grit or siliceous limestone, and occasionally containing concretions of phosphate of lime, which are largely worked for agricultural purposes.

V. White Chalk.—The top of the Upper Greensand becomes argillaceous, and passes up gradually into the base of the great formation known as the true Chalk, divided into the three subdivisions of the chalk-marl, white chalk without flints, and white chalk with flints. The first of these is simply argillaceous chalk, and passes up into a great mass of obscurely-stratified white chalk in which there are no flints (Turonien of D'Orbigny; Mittelquader of Germany). This, in turn, passes up into a great mass of white chalk, in which the stratification is marked by nodules of black flint arranged in layers (Senonien of D'Orbigny; Oberquader of Germany). The thickness of these three subdivisions taken together is sometimes over 1000 feet, and their geographical extent is very great. White Chalk, with its characteristic appearance, may be traced from the north of Ireland to the Crimea, a distance of about 1140 geographical miles; and, in an opposite direction, from the south of Sweden to Bordeaux, a distance of about 840 geographical miles.

VI. In Britain there occur no beds containing Chalk fossils, or in any way referable to the Cretaceous period, above the true White Chalk with flints. On the banks of the Maes, however, near Maestricht in Holland, there occurs a series of yellowish limestones, of about 100 feet in thickness, and undoubtedly superior to the White Chalk. These Maestricht beds (Danien of D'Orbigny) contain a remarkable series of fossils, the characters of which are partly Cretaceous and partly Tertiary. Thus, with the characteristic Chalk fossils, Belemnites, Baculites, Sea-Urchins, &c., are numerous Univalve Molluscs, such as Cowries and Volutes, which are otherwise exclusively Tertiary or Recent.

Holding a similar position to the Maestricht beds, and showing a similar intermixture of Cretaceous forms with later types, are certain beds which occur in the island of Seeland, in Denmark, and which are known as the Faxoee Limestone.

Of a somewhat later date than the Maestricht beds is the Pisolitic Limestone of France, which rests unconformably on the White Chalk, and contains a large number of Tertiary fossils along with some characteristic Cretaceous types.

The subjoined sketch-section exhibits the general succession of the Cretaceous deposits in Britain:—



In North America, strata of Lower Cretaceous age are well represented in Missouri, Wyoming, Utah, and in some other areas; but the greater portion of the American deposits of this period are referable to the Upper Cretaceous. The rocks of this series are mostly sands, clays, and limestones—Chalk itself being unknown except in Western Arkansas. Amongst the sandy accumulations, one of the most important is the so-called "marl" of New Jersey, which is truly a "Greensand," and contains a large proportion of glauconite (silicate of iron and potash). It also contains a little phosphate of lime, and is largely worked for agricultural purposes. The greatest thickness attained by the Cretaceous rocks of North America is about 9000 feet, as in Wyoming, Utah, and Colorado. According to Dana, the Cretaceous rocks of the Rocky Mountain territories pass upwards "without interruption into a coal-bearing formation, several thousand feet thick, on which the following Tertiary strata lie unconformably." The lower portion of this "Lignitic formation" appears to be Cretaceous, and contains one or more beds of Coal; but the upper part of it perhaps belongs to the Lower Tertiary. In America, therefore, the lowest Tertiary strata appear to rest conformably upon the highest Cretaceous; whereas in Europe, the succession at this point is invariably an unconformable one. Owing, however, to the fact that the American "Lignitic formation" is a shallow-water formation, it can hardly be expected to yield much material whereby to bridge over the great palaeontological gap between the White Chalk and Eocene in the Old World.

Owing to the fact that so large a portion of the Cretaceous formation has been deposited in the sea, much of it in deep water, the plants of the period have for the most part been found special members of the series, such as the Wealden beds, the Aix-la-Chapelle sands, and the Lignitic beds of North America. Even the purely marine strata, however, have yielded plant-remains, and some of these are peculiar and proper to the deep-sea deposits of the series. Thus the little calcareous discs termed "coccoliths," which are known to be of the nature of calcareous sea-weeds (Algoe) have been detected in the White Chalk; and the flints of the same formation commonly contain the spore-cases of the microscopic Desmids (the so-called Xanthidia), along with the siliceous cases of the equally diminutive Diatoms.

The plant-remains of the Lower Cretaceous greatly resemble those of the Jurassic period, consisting mainly of Ferns, Cycads, and Conifers. The Upper Cretaceous rocks, however, both in Europe and in North America, have yielded an abundant flora which resembles the existing vegetation of the globe in consisting mainly of Angiospermous Exogens and of Monocotyledons.[23] In Europe the plant-remains in question have been found chiefly in certain sands in the neighbourhood of Aix-la-Chapelle, and they consist of numerous Ferns, Conifers (such as Cycadopteris), Screw Pines (Pandanus), Oaks (Quercus), Walnut (Juglans), Fig (Ficus), and many Proteaceoe, some of which are referred to existing genera (Dryandra, Banksia, Grevillea, &c.)

[Footnote 23: The "Flowering plants" are divided into the two great groups of the Endogens and Exogens. The Endogens (such as Grasses, Palms, Lilies, &c.) have no true bark, nor rings of growth, and the stem is said to be "endogenous;" the young plant also possesses but a single seed-leaf or "cotyledon." Hence these plants are often simply called "Monocotyledons." The Exogens, on the other hand, have a true bark; and the stem increases by annual additions to the outside, so that rings of growth are produced. The young plant has two seed-leaves or "cotyledons," and these plants are therefore called "Dicotyledons." Amongst the Exogens, the Pines (Conifers) and the Cycads have seeds which are unprotected by a seed-vessel, and they are therefore called "Gymnosperms." All the other Exogens, including the ordinary trees, shrubs, and flowering plants, have the seeds enclosed in a seed-vessel, and are therefore called "Angiosperms." The derivation of these terms will be found in the Glossary at the end of the volume.]

In North America, the Cretaceous strata of New Jersey, Alabama, Nebraska, Kansas, &c., have yielded the remains of numerous plants, many of which belong to existing genera. Amongst these may be mentioned Tulip-trees (Liriodendron), Sassafras (fig. 186), Oaks (Quercus), Beeches (Fagus), Plane-trees (Platanus), Alders (Alnus), Dog-wood (Cornus), Willows (Salix), Poplars (Populus), Cypresses (Cupressus), Bald Cypresses (Taxodium), Magnolias, &c. Besides these, however, there occur other forms which have now entirely disappeared from North America—as, for example, species of Cinnamomum and Araucaria.

It follows from the above, that the Lower and Upper Cretaceous rocks are, from a botanical point of view, sharply separated from one another. The Palaeozoic period, as we have seen, is characterised by the prevalance of "Flowerless" plants (Cryptogams), its higher vegetation consisting almost exclusively of Conifers. The Mesozoic period, as a whole, is characterised by the prevalence of the Cryptogamic group of the Ferns, and the Gymnospermic groups of the Conifers and the Cycads. Up to the close of the Lower Cretaceous, no Angiospermous Exogens are certainly known to have existed, and Monocotyledonous plants or Endogens are very poorly represented. With the Upper Cretaceous, however, a new era of plant-life, of which our present is but the culmination, commenced, with a great and apparently sudden development of new forms. In place of the Ferns, Cycads, and Conifers of the earlier Mesozoic deposits, we have now an astonishingly large number of true Angiospermous Exogens, many of them belonging to existing types; and along with these are various Monocotyledonous plants, including the first examples of the great and important group of the Palms. It is thus a matter of interest to reflect that plants closely related to those now inhabiting the earth, were in existence at a time when the ocean was tenanted by Ammonites and Belemnites, and when land and sea and air were peopled by the extraordinary extinct Reptiles of the Mesozoic period.



As regards animal life, the Protozoans of the Cretaceous period are exceedingly numerous, and are represented by Foraminifera and Sponges. As we have already seen, the White Chalk itself is a deep-sea deposit, almost entirely composed of the microscopic shells of Foraminifers, along with Sponge-spicules, and organic debris of different kinds (see fig. 7). The green grains which are so abundant in several minor subdivisions of the Cretaceous, are also in many instances really casts in glauconite of the chambered shells of these minute organisms. A great many species of Foraminifera have been recognised in the Chalk; but the three principal genera are Globigerina, Rotalia (fig. 187), and Textularia—groups which are likewise characteristic of the "ooze" of the Atlantic and Pacific Oceans at great depths. The flints of the Chalk also commonly contain the shells of Foraminifera. The Upper Greensand has yielded in considerable numbers the huge Foraminifera described by Dr Carpenter under the name of Parkeria, the spherical shells of which are composed of sand-grains agglutinated together, and sometimes attain a diameter of two and a quarter inches. The Cretaceous Sponges are extremely numerous, and occur under a great number of varieties of shape and structure; but the two most characteristic genera are Siphonia and Ventriculites, both of which are exclusively confined to strata of this age. The Siphonioe (fig. 188) consist of a pear-shaped, sometimes lobed head, supported by a longer or shorter stern, which breaks up at its base into a number of root-like processes of attachment. The water gained access to the interior of the Sponge by a number of minute openings covering the surface, and ultimately escaped by a single, large, chimney-shaped aperture at the summit. In some respects these sponges present a singular resemblance to the beautiful "Vitreous Sponges" (Holtenia or Pheronema) of the deep Atlantic; and, like these, they were probably denizens of a deep sea, The Ventriculites of the Chalk (fig. 189) is, however, a genus still more closely allied to the wonderful flinty Sponges, which have been shown, by the researches of the Porcupine, Lightning, and Challenger expeditions, to live half buried in the Calcareous ooze of the abysses of our great oceans. Many forms of this genus are known, having "usually the form of graceful vases, tubes, or funnels, variously ridged or grooved, or otherwise ornamented on the surface, frequently expanded above into a cup-like lip, and continued below into a bundle of fibrous roots. The minute structure of these bodies shows an extremely delicate tracery of fine tubes, sometimes empty, sometimes filled with loose calcareous matter dyed with peroxide of iron."—(Sir Wyville Thomson.) Many of the Chalk sponges, originally calcareous, have been converted into flint subsequently; but the Ventriculites are really composed of this substance, and are therefore genuine "Siliceous Sponges," like the existing Venus's Flower-Basket (Euplectella). Like the latter, the skeleton was doubtless originally composed, in the young state, of disconnected six-rayed spicules, which ultimately become fixed together to constitute a continuous frame-work. The sea-water, as in the recent forms, must have been admitted to the interior of the Sponge by numerous apertures on its exterior, subsequently escaping by a single large opening at its summit.



Amongst the Coelenterates, the "Hydroid Zoophytes" are represented by a species of the encrusting genus Hydractinia, the horny polypary of which is so commonly found at the present day adhering to the exterior of shells. The occurrence of this genus is of interest, because it is the first known instance in the entire geological series of the occurrence of an unquestionable Hydroid of a modern type, though many of the existing forms of these animals possess structures which are perfectly fitted for preservation in the fossil condition. The corals of the Cretaceous series are not very numerous, and for the most part are referable to types such as Trochocyathus, Stephanophyllia, Parasmilia, Synhelia (fig. 190), &c., which belong to the same great group of corals as the majority of existing forms. We have also a few "Tabulate Corals" (Polytremacis), hardly, if at all, generically separable from very ancient forms (Heliolites); and the Lower Greensand has yielded the remains of the little Holocystis elegans, long believed to be the last of the great Palaeozoic group of the Rugosa.



As regards the Echinoderms, the group of the Crinoids now exhibits a marked decrease in the number and variety of its types. The "stalked" forms are represented by Pentacrinus and Bourgueticrinus, and the free forms by Feather-stars like our existing Comatuloe; whilst a link between the stalked and free groups is constituted by the curious "Tortoise Encrinite (Marsupites). By far the most abundant Cretaceous Echinoderms, however, are Sea-urchins (Echinoids); though several Star-fishes are known as well. The remains of Sea-urchins are so abundant in various parts of the Cretaceous series, especially in the White Chalk, and are often so beautifully preserved, that they constitute one of the most marked features of the fauna of the period. From the many genera of Sea-urchins which occur in strata of this age, it is difficult to select characteristic types; but the genera Galerites (fig. 191), Discoidea (fig. 192), Micraster, Ananchytes, Diadema, Salenia, and Cidaris, may be mentioned as being all important Cretaceous groups.

Coming to the Annulose Animals of the Cretaceous period, there is little special to remark. The Crustaceans belong for the most part to the highly-organised groups of the Lobsters and the Crabs (the Macrurous and Brachyurous Decapods); but there are also numerous little Ostracodes, especially in the fresh-water strata of the Wealden. It should further be noted that there occurs here a great development of the singular Crustaceous family of the Barnacles (Lepadidoe), whilst the allied family of the equally singular Acorn-shells (Balanidoe) is feebly represented as well.



Passing on to the Mollusca, the class of the Sea-mats and Sea-mosses (Polyzoa) is immensely developed in the Cretaceous period, nearly two hundred species being known to occur in the Chalk. Most of the Cretaceous forms belong to the family of the Escharidoe, the genera Eschara and Escharina (fig. 193) being particularly well represented. Most of the Cretaceous Polyzoans are of small size, but some attain considerable dimensions, and many simulate Corals in their general form and appearance.

The Lamp-shells (Brachiopods) have now reached a further stage of the progressive decline, which they have been undergoing ever since the close of the Palaeozoic period. Though individually not rare, especially in certain minor subdivisions of the series, the number of generic types has now become distinctly diminished, the principal forms belonging to the genera Terebratula, Terebratella (fig. 194), Terebratulina, Rhynchonella, and Crania (fig. 195). In the last mentioned of these, the shell is attached to foreign bodies by the substance of one of the valves (the ventral), whilst the other or free valve is more or less limpet-shaped. All the above-mentioned genera are in existence at the present day; and one species—namely, Terebratulina striata—appears to be undistinguishable from one now living—the Terebratulina caputserpentis.



Whilst the Lamp-shells are slowly declining, the Bivalves (Limellibranchs) are greatly developed, and are amongst the most abundant and characteristic fossils of the Cretaceous period. In the great river-deposit of the Wealden, the Bivalves are forms proper to fresh water, belonging to the existing River-mussels (Unio), Cyrena and Cyclas; but most of the Cretaceous Lamellibranchs are marine. Some of the most abundant and characteristic of these belong to the great family of the Oysters (Ostreidoe). Amongst these are the genera Gryphtoea and Exogyra, both of which we have seen to occur abundantly in the Jurassic; and there are also numerous true Oysters (Ostrea, fig. 196) and Thorny Oysters (Spondylus, fig. 197). The genus Trigonia, so characteristic of the Mesozoic deposits in general, is likewise well represented in the Cretaceous strata. No single genus of Bivalves is, however, so highly characteristic of the Cretaceous period as Inoceramus, a group belonging to the family of the Pearl-mussels (Aviculidoe). The shells of this genus (fig. 198) have the valves unequal in size, the larger valve often being much twisted, and both valves being marked with radiating ribs or concentric furrows. The hinge-line is long and straight, with numerous pits for the attachment of the ligament which serves to open the shell. Some of the Inocerami attain a length of two or three feet, and fragments of the shell are often found perforated by boring Sponges. Another extraordinary family of Bivalves, which is exclusively confined to the Cretaceous rocks, is that of the Hippuritidoe. All the members of this group (fig. 199) were attached to foreign objects, and lived associated in beds, like Oysters. The two valves of the shell are always altogether unlike in sculpturing, appearance, shape, and size; and the cast of the interior of the shell is often extremely unlike the form of the outer surface. The type-genus of the family is Hippurites itself (fig. 199), in which the shell is in the shape of a straight or slightly-twisted horn, sometimes a foot or more in length, constituted by the attached lower valve, and closed above by a small lid-like free upper valve. About a hundred species of the family of the Hippuritidoe are known, all of these being Cretaceous, and occurring in Britain (one species only), in Southern Europe, the West Indies, North America, Algeria, and Egypt. Species of this family occur in such numbers in certain compact marbles in the south of Europe, of the age of the Upper Cretaceous (Lower Chalk), as to have given origin to the name of "Hippurite Limestones," applied to these strata.



The Univalves (Gasteropods) of the Cretaceous period are not very numerous, nor particularly remarkable. Along with species of the persistent genus Pleurotomaria and the Mesozoic Nerinoea, we meet with examples of such modern types as Turritella and Natica, the Staircase-shells (Solarium), the Wentle-traps (Scalaria), the Carrier-shells (Phorus), &c. Towards the close of the Cretaceous period, and especially in such transitional strata as the Maestricht beds, the Faxoee Limestone, and the Pisolitic Limestone of France, we meet with a number of carnivorous ("siphonostomatous") Univalves, in which the mouth of the shell is notched or produced into a canal. Amongst these it is interesting to recognise examples of such existing genera as the Volutes (Voluta, fig. 200), the Cowries (Cyproea), the Mitre-shells (Mitra), the Wing - shells (Strombus), the Scorpion-shells (Pteroceras), &c.



Upon the whole, the most characteristic of all the Cretaceous Molluscs are the Cephalopods, represented by the remains of both Tetrabranchiate and Dibranchiate forms. Amongst the former, the long-lived genus Nautilus (fig. 201) again reappears, with its involute shell, its capacious body-chamber, its simple septa between the air-chambers, and its nearly or quite central siphuncle. The majority of the chambered Cephalopods of the Cretaceous belong, however, to the complex and beautiful family of the Ammonitidoe, with their elaborately folded and lobed septa and dorsally-placed siphuncle. This family disappears wholly at the close of the Cretaceous period; but its approaching extinction, so far from being signalised by any slow decrease and diminution in the number of specific or generic types, seems to have been attended by the development of whole series of new forms. The genus Ammonites itself, dating from the Carboniferous, has certainly passed its prime, but it is still represented by many species, and some of these attained enormous dimensions (two or three feet in diameter). The genus Ancyloceras (fig. 202), though likewise of more ancient origin (Jurassic), is nevertheless very characteristic of the Cretaceous. In this genus the first portion of the shell is in the form of a flat spiral, the coils of which are not in contact; and its last portion is produced at a tangent, becoming ultimately bent back in the form of a crosier. Besides these pre-existent types, the Cretaceous rocks have yielded a great number of entirely new forms of the Ammonitidoe, which are not known in any deposits of earlier or later date. Amongst the more important of these may be mentioned Crioceras, Turrilites, Scaphites, Hamites, Ptychoceras, and Baulites. In the genus Crioceras (fig. 204, d), the shell consists of an open spiral, the volutions of which are not in contact, thus resembling a partially-unrolled Ammonite or the inner portion of an Ancyloceras. In Turrilites (fig. 203), the shell is precisely like that of the Ammonite in its structure; but instead of forming a flat spiral, it is coiled into an elevated turreted shell, the whorls of which are in contact with one another. In the genus Scaphites (fig. 204, e), the shell resembles that of Ancyloceras in consisting of a series of volutions coiled into a flat spiral, the last being detached from the others, produced, and ultimately bent back in the form of a crosier; but the whorls of the enrolled part of the shell are in contact, instead of being separate as in the latter. In the genus Hamites (fig. 204, f), the shell is an extremely elongated cone, which is bent upon itself more than once, in a hook-like manner, all the volutions being separate. The genus Ptychoteras (fig. 204, a) is very like Hamites, except that the shell is only bent once; and the two portions thus bent are in contact with one another. Lastly, in the genus Baculites (fig. 204, b and c) the shell is simply a straight elongated cone, not bent in any way, but possessing the folded septa which characterise the whole Ammonite family. The Baculite is the simplest of all the forms of the Ammonitidoe; and all the other forms, however complex, may be regarded as being simply produced by the bending or folding of such a conical septate shell in different ways. The Baculite, therefore, corresponds, in the series of the Ammonitidoe, to the Orthoceras in the series of the Nautilidoe. All the above-mentioned genera are characteristically, or exclusively, Cretaceous, and they are accompanied by a number of other allied forms, which cannot be noticed here. Not a single one of these genera, further, has hitherto been detected in any strata higher than the Cretaceous. We may therefore consider that these wonderful, varied, and elaborate forms of Ammonitidoe constitute one of the most conspicuous features in the life of the Chalk period.



The Dibranchiate Cephalopods are represented partly by the beak-like jaws of unknown species of Cuttle-fishes and partly by the internal skeletons of Belemnites. Amongst the latter, the genus Belemnites itself holds its place in the lower part of the Cretaceous series; but it disappears in the upper portion of the series, and its place is taken by the nearly-allied genus Belemnitella (fig. 205), distinguished by the possession of a straight fissure in the upper end of the guard. This also disappears at the close of the Cretaceous period; and no member of the great Mesozoic family of the Belemnitidoe has hitherto been discovered in any Tertiary deposit, or is known to exist at the present day.



Passing on next to the Vertebrate Animals of the Cretaceous period, we find the Fishes represented as before by the Ganoids and the Placoids, to which, however, we can now add the first known examples of the great group of the Bony Fishes or Teleosteans, comprising the great majority of existing forms. The Ganoid fishes of the Cretaceous (Lepidotus, Pycnodus, &c.) present no features of special interest. Little, also, need be said about the Placoid fishes of this period. As in the Jurassic deposits, the remains of these consist partly of the teeth of genuine Sharks (Lamna, Odontaspis, &c.) and partly of the teeth and defensive spines of Cestracionts, such as the living Port-Jackson Shark. The pointed and sharp-edged teeth of true Sharks are very abundant in some beds, such as the Upper Greensand, and are beautifully preserved. The teeth of some forms (Carcharias, &c.) attain occasionally a length of three or four inches, and indicate the existence in the Cretaceous seas of huge predaceous fishes, probably larger than any existing Sharks. The remains of Cestracionts consist partly of the flattened teeth of genera such as Acrodus and Ptychodus (the latter confined to rocks of this age), and partly of the pointed teeth of Hybodus, a genus which dates from the Trias. In this genus the teeth (fig. 206) consist of a principal central cone, flanked by minor lateral cones; and the fin-spines (fig. 207) are longitudinally grooved, and carry a series of small spines on their hinder or concave margin. Lastly, the great modern order of the Bony Fishes or Teleosteans makes its first appearance in the Upper Cretaceous rocks, where it is represented by forms belonging to no less than three existing groups—namely, the Salmon family (Salmonidoe), the Herring family (Clupeidoe), and the Perch family (Percidoe). All these fishes have thin, horny, overlapping scales, symmetrical ("homocercal") tails, and bony skeletons. The genus Beryx (fig. 208, 1) is one represented by existing species at the present day, and belongs to the Perch family. The genus Osmeroides, again (fig. 208, 2), is supposed to be related to the living Smelts (Osmerus), and, therefore, to belong to the Salmon tribe.



No remains of Amphibians have hitherto been detected in any part of the Cretaceous series; but Reptiles are extremely numerous, and belong to very varied types. As regards the great extinct groups of Reptiles which characterise the Mesozoic period as a whole, the huge "Enaliosaurs" or "Sea-Lizards" are still represented by the Ichthyosaur and the Plesiosaur. Nearly allied to the latter of these is the Elasmosaurus of the American Cretaceous, which combined the long tail of the Ichthyosaur with the long neck of the Plesiosaur. The length of this monstrous Reptile could not have been less than fifty feet, the neck consisting of over sixty vertebrae and measuring over twenty feet in length. The extraordinary Flying Reptiles of the Jurassic are likewise well represented in the Cretaceous rocks by species of the genus Pterodactylus itself, and these later forms are much more gigantic in their dimensions than their predecessors. Thus some of the Cretaceous Pterosaurs seem to have had a spread of wing of from twenty to twenty-five feet, more than realising the "Dragons" of fable in point of size. The most remarkable, however, of the Cretaceous Pterosaurs are the forms which have recently been described by Professor Marsh under the generic title of Pteranodon. In these singular forms—so far only known as American—the animal possessed a skeleton in all respects similar to that of the typical Pterodactyles, except that the jaws are completely destitute of teeth. There is, therefore, the strongest probability that the jaws were encased in a horny sheath, thus coming to resemble the beak of a Bird. Some of the recognised species of Pteranodon are very small; but the skull of one species (P. Longiceps) is not less than a yard in length, and there are portions of the skull of another species which would indicate a length of four feet for the cranium. These measurements would point to dimensions larger than those of any other known Pterosaurs.

The great Mesozoic order of the Deinosaurs is largely represented in the Cretaceous rocks, partly by genera which previously existed in the Jurassic period, and partly by entirely new types. The great delta-deposit of the Wealden, in the Old World, has yielded the remains of various of these huge terrestrial Reptiles, and very many others have been found in the Cretaceous deposits of North America. One of the most celebrated of the Cretaceous Deinosaurs is the Iguanodon, so called from the curious resemblance of its teeth to those of the existing but comparatively diminutive Iguana. The teeth (fig. 209) are soldered to the inner face of the jaw, instead of being sunk in distinct sockets; and they have the form of somewhat flattened prisms, longitudinally ridged on the outer surface, with an obtusely triangular crown, and having the enamel crenated on one or both sides. They present the extraordinary feature that the crowns became worn down flat by mastication, showing that the Iguanodon employed its teeth in actually chewing and triturating the vegetable matter on which it fed. There can therefore be no doubt but that the Iguanodon, in spite of its immense bulk, was an herbivorous Reptile, and lived principally on the foliage of the Cretaceous forests amongst which it dwelt. Its size has been variously estimated at from thirty to fifty feet, the thigh-bone in large examples measuring nearly five feet in length, with a circumference of twenty-two inches in its smallest part. With the strong and massive hind-limbs are associated comparatively weak and small fore-limbs; and there seems little reason to doubt that the Iguanodon must have walked temporarily or permanently upon its hind-limbs, after the manner of a Bird. This conjecture is further supported by the occurrence in the strata which contain the bones of the Iguanodon of gigantic three-toed foot-prints, disposed singly in a double track. These prints have undoubtedly been produced by some animal walking on two legs; and they can hardly, with any probability, be ascribed to any other than this enormous Reptile. Closely allied to the Iguanodon is the Hadrosaurus of the American Cretaceous, the length of which is estimated at twenty-eight feet. Iguanodon does not appear to have possessed any integumentary skeleton; but the great Hyloeosaurus of the Wealden seems to have been furnished with a longitudinal crest of large spines running down the back, similar to that which is found in the comparatively small Iguanas of the present day. The Megalosaurus of the Oolites continued to exist in the Cretaceous period; and, as we have previously seen, it was carnivorous in its habits. The American Loelaps was also carnivorous, and, like the Megalosaur, which it very closely resembles, appears to have walked upon its hind-legs, the fore-limbs being disproportionately small.