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The primitive biconcave vertebrae of the fish and of the early land vertebrates were retained, and the limbs degenerated into short paddles. The skin of the ichthyosaur was smooth like that of a whale, and its food was largely fish and cephalopods, as the fossil contents of its stomach prove.
In fine, the discovery of young developed skeletons inside the adult frames has taught us that the Ichthgosaur had become viviparous, like the mammal. Cutting its last connection with the land, on which it originated it ceased to lay eggs, and developed the young within its body. The Ichthyosaur came of the reptile group which we have called the Diapsids.
The rest of the body may then be drifted elsewhere, and sometimes may be swallowed entire by a predaceous reptile or fish, such as an ichthyosaur or a shark."
Even now they afford a most interesting page in the story of evolution, and their total disappearance from the face of the earth in the next geological period will not be unintelligible. We turn from them to the remaining orders of the Jurassic reptiles. In the popular mind, perhaps, the Ichthyosaur and Plesiosaur are the typical representatives of that extinct race.
One fossilised Ichthyosaur of the weaker-toothed variety has been found with the remains of two hundred Belemnites in its stomach. It is a flash of light on the fierce struggle and carnage which some recent writers have vainly striven to attenuate. The eyes, again, which may in the larger animals be fifteen inches in diameter, are protected by a circle of radiating bony plates.
Warmed and well fed, his eyes half-sheathed in their membraneous lids, he gazed out vacantly across the waving herbage of the shallows, across the slow, pale tides whose surface boiled from time to time above the rush of some unseen giant of a shark or ichthyosaur. In the heavy heat of the afternoon the young world had become very still.
The latter groups soon became extinct, but the former continued for some millions of years, and became remarkably adapted to marine life, like the whale at a later period. The Ichthyosaur of the Jurassic is a remarkably fish-like animal.
It is supposed to have lived in shallow seas and estuaries, and to have breathed air like the Ichthyosaur and our modern cetacea. One specimen of Ichthyosaurus platydon, from the Lias at Lyme, now in the British Museum, must have belonged to an animal more than 24 feet in length; and there are species of Plesiosaurus which measure from 18 to 20 feet in length.
"Man in his natural state," said Cortlandt, "would have but small chance of surviving long among such neighbours. Buckland, I think, once indulged in the jeu d'esprit of supposing an ichthyosaur lecturing on the human skull. 'You will at once perceive, said the lecturer, 'that the skull before us belonged to one of the lower order of animals.
The Crocodilians also appear in the later Triassic, abound in the Jurassic, and give way before the later types, the true Crocodiles, in the Cretaceous. They were marine animals with naked skin, a head and neck something like that of the Ichthyosaur, and paddles like those of the Plesiosaur.
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