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 Rhætic 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 Kimméridgien_ 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 _débris_ 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 Palæozoic 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 à 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 à Nérinées." 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 Palæozoic 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 striæ 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 (vertebræ) also show the same adaptation to an aquatic mode of life, being hollowed out at both ends, like the biconcave vertebræ 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 vertebræ. The bodies of the vertebræ, 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 vertebræ 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 vertebræ; whereas in all existing Birds the tail is abbreviated, and the terminal vertebræ 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 vertebræ, 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 vertebræ. 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 vertebræ, 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 vertebræ, 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 vertebræ, each of which supports a single pair of quill-feathers. In the flying Birds of the present day, as before mentioned, the terminal vertebræ 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 vertebræ, 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 palæontologist 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 Asteriadæ' (Palæontographical 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 Flötzgebirge Württembergs.' Quenstedt. (11) 'Jura Formation.' Oppel. (12) 'Paléontologie du Département de la Moselle.' Terquem. (13) 'Cours élémentaire de Paléontologie.' D'Orbigny. (14) 'Paléontologie Française.' D'Orbigny. (15) 'Fossil Echinodermata of the Oolitic Formation' (Palæontographical Society). Wright. (16) 'Brachiopoda of the Oolitic Formation' (Palæontographical Society). Davidson. (17) 'Mollusca of the Great Oolite' (Palæontographical Society). Morris and Lycett. (18) 'Monograph of the Fossil Trigoniæ' (Palæontographical Society). Lycett. (19) 'Corals of the Oolitic Formation' (Palæontographical Society). Edwards and Haime. (20) 'Supplement to the Corals of the Oolitic Formation' (Palæontographical Society). Martin Duncan. (21) 'Monograph of the Belemnitidæ' (Palæontographical Society). Phillips. (22) 'Structure of the Belemnitidæ' (Mem. Geol. Survey). Huxley. (23) 'Sur les Belemnites.' Blainville. (24) 'Cephalopoden.' Quenstedt. (25) 'Mineral Conchology.' Sowerby. (26) 'Jurassic Cephalopoda' (Palæontologica 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) 'Beiträge zur Petrefaktenkunde.' Münster. (34) 'Petrefacta Germaniæ.' Goldfuss. (35) 'Lethæa Rossica.' Eichwald. (36) 'Fossil Fishes' (Decades of the Geol. Survey). Sir Philip Egerton. (37) 'Manual of Palæontology.' Owen. (38) 'British Fossil Mammals and Birds.' Owen. (39) 'Monographs of the Fossil Reptiles of the Oolitic Formation' (Palæontographical Society). Owen. (40) 'Fossil Mammals of the Mesozoic Formations' (Palæontographical Society). Owen. (41) 'Catalogue of Ornithosauria.' Seeley. (42) "Classification of the Deinosauria"--'Quart. Journ. Geol. Soc.,' vol. xxvi., 1870. Huxley.