The Ancient Life History of the Earth Part 18

[Ill.u.s.tration: Fig. 170.--_Ammonites Humphresia.n.u.s_. Inferior Oolite.]

[Ill.u.s.tration: Fig. 171.--_Ammonites bifrons_. Lias.]

Lastly, the order of the _Cephalopoda_, in both its Tetrabranchiate and Dibranchiate sections, undergoes a vast development in the Jura.s.sic 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 _Goniat.i.tes_ 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 sh.e.l.l 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 sh.e.l.l 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 sh.e.l.l; but the princ.i.p.al character which distinguishes _Ammonites_ from _Goniat.i.tes_ and _Cerat.i.tes_ is the wonderfully complex manner in which the _septa_, or part.i.tions 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 sh.e.l.l-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 a.s.sociated 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.

[Ill.u.s.tartion: Fig. 172.--_Beloteuthis subcostata_ Jura.s.sic (Lias).]

The Cuttle-fishes or _Dibranchiate Cephalopods_ const.i.tute a feature in the life of the Jura.s.sic period little less conspicuous and striking than that afforded by the mult.i.tudinous and varied chambered sh.e.l.ls 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 sh.e.l.l 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 Jura.s.sic period, in the shape of the fossilised jaws or beak, the ink-bag, and, most commonly of all, the h.o.r.n.y 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 t.i.tle of "Rhyncholites;" and in their form and function they resemble the h.o.r.n.y, 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 h.o.r.n.y 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 h.o.r.n.y 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 Jura.s.sic 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 sh.e.l.l (the "phragmacone"), the part.i.tions between the chambers of which are pierced by a marginal tube or "siphuncle." This conical sh.e.l.l--curiously similar in its structure to the _external_ sh.e.l.l of the Nautilus--is extended forwards into a h.o.r.n.y "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 Jura.s.sic 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 Jura.s.sic 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 h.o.r.n.y hooks; there was a large ink-sac; and the mouth was armed with h.o.r.n.y mandibles resembling in shape the beak of a parrot.

[Ill.u.s.tration: Fig. 173.--A, Restoration of the animal of the Belemnite; B, Diagram showing the complete skeleton of a Belemnite, consisting of the chambered phragmacone (a), the guard (b), and the h.o.r.n.y pen (c); C, Specimen of _Belemnites ca.n.a.liculatus_, from the Inferior Oolite. (After Phillips.)]

[Ill.u.s.tration: Fig. 174.--_Tetragonolepis (restored), and scales of the same. Lias.]

Coming next to the _Vertebrates_, we find that the Jura.s.sic _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 ("h.o.m.ocercal") 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 (_Notida.n.u.s_) 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 sh.e.l.l-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.

[Ill.u.s.tration: Fig. 175.--Tooth of _Acrodus n.o.bilis_. Lias.]

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 Jura.s.sic age.

[Ill.u.s.tration: Fig. 176.--_Ichthyosaurus communis. Lias.]

Much more important than the Fishes of the Jura.s.sic 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, const.i.tuting 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 pa.s.s 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 sh.o.r.e, 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 Jura.s.sic seas.

[Ill.u.s.tration: Fig. 177.--_Plesiosaurus dolichodeirus_, restored.

Lias.]

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 dest.i.tute of any scaly or bony invest.i.ture. 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 a.s.sociated; that it may have occasionally visited the sh.o.r.e, 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 a.s.saults 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.

[Ill.u.s.tration: Fig. 178.--_Pterodactylus cra.s.sirostis_. From the Lithographic Slates of Solenhofen (Middle Oolite). The figure is "restored," and it seems certain that the restoration is incorrect in the comparatively unimportant particular, that the hand should consist of no more than four fingers, three short and one long, instead of five, as represented.]

Another extraordinary group of Jura.s.sic 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 Jura.s.sic, and it is a.s.sociated 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 cla.s.s by themselves. The chief points which separate them from Birds, as a cla.s.s, 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.

[Ill.u.s.tration: Fig. 179--_Rhamphorhynchus Bucklandi_, restored.

Bath Oolite, England. (After the late Professor Phillips.)]

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 Lia.s.sic, 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 const.i.tuted a h.o.r.n.y beak. Like most of the other Jura.s.sic 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 sh.o.r.e, 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 a.n.a.logy of structure and probable way of life would lead us to a.s.similate 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 circ.u.mstances, may be held to claim for the Stonesfield fossil a more than fanciful similitude to the groups of Cormorants, and other marine divers, which const.i.tute an effective part of the picturesque army of robbers of the sea."

Another extraordinary and interesting group of the Mesozoic Reptiles is const.i.tuted 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 cla.s.s, being in some respects intermediate between the proper Reptiles and the great wingless Birds, like the Ostrich and Ca.s.sowary. 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 Jura.s.sic 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.

[Ill.u.s.tration: Fig. 180.--Skull of _Megalosaurus_, on a scale one-tenth of nature. Restored. (After Professor Phillips.)]

_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 Suss.e.x, in fluviatile acc.u.mulations. 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 cib.u.m_.

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 Jura.s.sic Reptiles which we have hitherto been considering are wholly unrepresented at the present day, and do not even pa.s.s 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 cla.s.s-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 Tria.s.sic 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 Jura.s.sic; 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.

[Ill.u.s.tration: Fig. 181.--_Archoeopteryx macrura_, showing tail and tail-feathers, with detached bones. Reduced. From the Lithographic Slate of Solenhofen.]

[Ill.u.s.tration: Fig. 182.--Restoration of _Archoeopteryx macrura_.

(After Owen.)]

The great cla.s.s 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 cla.s.s 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 cl.u.s.ter 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 cla.s.s 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.

[Ill.u.s.tration: Fig. 183.--Lower jaw of _Amphitherium_ (_Thylacotherium_) _Prevostii_. Stonesfield Slate (Great Oolite.)]

[Ill.u.s.tration: Fig. 184. Oolitic Mammals.--1, Lower jaw and teeth of _Phascolotherium_, Stonesfield Slate; 2, Lower jaw and teeth of _Amphitherium_, Stonesfield Slate; 3, Lower jaw and teeth of _Triconodon_, Purbeck beds; 4, Lower jaw and teeth of _Plagiaulax_, Purbeck beds. All the figures are of the natural size.]

The _Mammals_ of the Jura.s.sic 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 Jura.s.sic 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 Jura.s.sic 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 Tria.s.sic, nevertheless extended its range into the Jura.s.sic. Upon the whole, therefore, there is reason to conclude that Australia has undergone since the close of the Jura.s.sic 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 Jura.s.sic 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.