Imagens das páginas
PDF
ePub

consisted of a flattened tube, twisted into a spiral or corkscrew form, and reduced to the smallest possible dimensions.

Judging from the existing reptiles, and the gigantic sauroid fishes of the older period, it might perhaps have been anticipated that the marine saurians, of which the Ichthyosaurus is one of the most important genera, should be provided with hard bony plates, or scutes, as they are called, at once enclosing and defending the animal. The absence of such scutes among the fossils of the lias, which has not only retained and handed down all the hard parts in great abundance and perfection, but exhibits even some of the softer portions of the body, would alone render this questionable. But the doubt has been completely justified by the discovery of actual portions of the undefended skin, which, preserving the analogy with the cetaceans, appears to have been naked, of considerable thickness, and covered with minute folds and wrinkles on the belly, instead of scales.

Fig. 64

ICHTHYOSAURUS.

(Restored Outline.)

On the whole, then, the Ichthyosaurus may be described as an air-breathing reptile, which sometimes attained a length of thirty to forty feet, which was covered like a whale with smooth, naked, thick skin, and

whose place of ordinary habitation was the open sea. Its head was large and somewhat like that of the dolphin, but its general form, no less than the particular contrivances of the jaws and teeth, were truly crocodilian. Associated with this crocodilian head, we find enormous eagle-like eyes, carefully defended and made admirably efficient by an apparatus of bony scales, permitting distant vision in the air, near the surface of the water, and in the dim abysses of the ocean. The body of this creature was perfectly flexible and fish-like, but, instead of fins, it had two pair of very powerful paddles, permitting of the utmost freedom of motion in swimming, and forming not inconvenient limbs to assist in locomotion on land. A large and efficient vertical tail completed this strange mixture of fish, reptile, and whale, and, though no living representative exists not merely of its genus, but even of the great natural order to which it belongs, it once played no unimportant part in extensive tracts of ocean, which soon after the commencement of the secondary period covered that part of the northern hemisphere now occupied by the continent and islands of Europe.

Towards the close of the deposit of the great mass of red sandstone and marl which immediately rests on the paleozoic rocks, these beds seem to have gradually changed their character, the marl preponderating and becoming more calcareous. Although sandy and calcareous mud was still deposited uniformly, abundantly, and very widely, we have in the beds of mud thus preserved no distinct indications of the vicinity of land, for the fragments of wood that occur are almost invariably covered with marine

animals, proving that they had floated or been drifted far out to sea. The change in the sea-bottom, and the lapse of time, accompanied by the upheaval or sinking of land, produced the effect of destroying many old species, while other new ones entered the field. These exhibited an undoubted approximation to the animals of a later period, and include some very curious star-fishes, several crustaceans, and a large number of mollusca or shell-fish, both bivalve and univalve, all having analogies with existing species, though all specifically different. Of these the most remarkable in their departure from the existing type are the Pentacrinite, the Terebratulæ among bivalve shells, and the ammonite and belemnite among the univalves. The fishes and the reptiles exhibit still more prominently the differences that then existed.

If we wish to pass in review the various groups most characteristic of this singular period, concerning whose natural history we have so many and such distinct facts recorded, we must imagine a wide tract of open sea, into which a quantity of fine sediment of calcareous mud was in some way carried and deposited.* From the distant land whence this mud was washed came also occasionally trunks of trees conveyed by marine or river currents. Attached to them, and also occasionally fastened to sea-weeds or other floating bodies, would appear in large clusters-(like the bunches of barnacles sometimes suspended from a ship's bottom)—the singular pentacrinites, their long stony column fringed thickly with branches of articulated stone, with a stony coat of mail surrounding the Such a deposit is probably now going on in the Yellow Sea, off the coasts of China.

pouch or stomach, and a similar but more delicate defence covering the extensile proboscis. With innumerable arms widely extended in a complicated fringe, this strange mass of living stone expanded itself, and drew within its cold embrace the floating bodies on which it fed. One might fancy that some marine Briareus, looking on the strife and carnage of this great reptilian period, whose horrors might well have had the fabled effect attributed to the snakes of Medusa's head, had suddenly become petrified, retaining however its vital powers, and, with its complicated skeleton, continued to perform its office by cleansing the sea of an accumulation of decaying animal matter.

But while the Pentacrinite was thus the floating scavenger of that period, the bottom of the sea, although not covered with encrinites and corals, was well provided with other animals performing the same part in nature. The great beds of Gryphea—the oysters of their day-are sufficient proof of this, and the Terebratulæ and Spirifers tell the same tale. Among the invertebrate animals, however, the ammonite and the belemnite were undoubtedly the most remarkable, and, at least in certain districts of the sea, were enormously abundant. Some of them being enclosed in shells, some enclosing shells, and some perhaps not provided with any solid framework, swam about, or dwelt at various depths, and by their carnivorous and voracious habits greatly tended to keep down the exuberance of the lower forms of life.

The neighbourhood of the shore, and the shallow banks during this period were peopled by multitudes of fishes of moderate size, living chiefly on the crabs, lobsters, and shell-fish, or on the encrinital animals;

and, for the purpose of crushing the shells of such creatures, these fishes were provided with a pavement of hard rough enamelled teeth fixed on the palate. The whole body also and the head were covered with plates of bone, also coated with enamel, and serving as a defence against the attack of the larger ammonites and belemnites. Farther out at sea were tribes of sharks of different species, all predaceous and carnivorous, and many of them of the most gigantic proportions. No fishes like those now common on the coasts of England then existed on the earth.

The fishes, though abundant and represented by a powerful and important group, had ceased to be the lords of creation in the lias seas. The depths of the sea as well as the shallows, the broad expanse of waters as well as the coast-line, were in those days the dwelling-places of a group of reptiles of which every representative has long since become extinct.

Two of these have more especially attracted attention, in consequence of their great abundance in the fossil state in our own country, but they are by no means the only ones known. Of these two, one was more exclusively tenant of the deep, while the other was probably more frequently met with on the mud banks or on shore. Both were truly marine in their habits, and both seem to have served as the representatives of the great cetacean tribe-the whales, the porpoises, and other similar animals-now existing.

It is difficult to imagine, without appearing to caricature, the conditions of existence of such animals. We know indeed their form, their proportions, their strange contrivances of structure, their very skin, and

[ocr errors]
« AnteriorContinuar »