ICHTHYOSAITRUS (Gr. fish-reptile), a remarkable genus of reptiles which inhabited the sea during the deposition of the secondary strata. Like the modern cetacea, their structure was modified to suit their aquatic life. The body was shaped like that of fish, the limbs were developed into paddles, and the tail, long and lizard-like, was furnished, it is believed, with a fleshy fin, as in the dolphin, except that its position was vertical. The head was large, and produced into a long and pointed snout, resembling that of the crocodile, except that the orbit was much larger, and had the nostril placed close to it, as in the whale, and not near the end of the snout. The jaws were furnished with a large series of powerful conical teeth, lodged close together in a continuous groove, in which the divisions for sockets, which exist in the crocodile, were indicated by the vertical ridges on the maxillary bone. The teeth were hollow at the root, sheathing the young teeth, which gradually absorbed the base of the older ones, and, as they grew, pressed them forward, until they finally displaced them. The long and' slender jaws were strengthened to resist any sudden shock by being formed of many thin bony plates, which produced light and elastic as well as strong jaws. The most remarkable feature in the head was the eye, which was not only very large—in some specimens measuring 13 in. in diameter—but was specially fitted to accommodate itself for vision in air or water, as well as for speedily altering the focal distance while pursuing its prey. The structure, which thus fitted the eye so remarkably to the wants of the animal, consists of a circle of 13 or more overlapping sciorotic bony plates sur rounding the pupil, as in birds. This circle acted as a sort of self-adjustin, telescope, :tad, accompanied by the extraordinary amount of light admitted by the large pupil, enabled the ichthyosaurus to discover its prey at great or little distances in the obscurity of the night, and in the depths of the sea. '1 he neck was so short that the body was
probably not in the least constricted behind the head. The backbone was fish like; each joint had both its surfaces hollow, the whole column very flexible. The small size of the paddles compared with the body, and the stiffness of the short neck, seem to suggest that the tail must have been an important organ of motion. Prof. Owen is satisfied that it was furnished with a vertical tail, because the vertebrae are compressed vertically, and also because the tail is frequently found disarticulated short distance from its extremity, as if the weight of the upright tail had caused it to fall when the animal had begun to decompose. The fish-like body, the four paddles, and especially the powerful tail, would make the iehthyosauri active in their move ments; and consequently, with their predaceous habits, very dangerous enemies to the other animals that inhabited with them the secondary seas. That their principal food consisted of fishes, is evident from the masses of broken bones and scales of con temporary fishes that have been found under their ribs in the place where the stomach of the animal was situated.
The remains of ichthyosauri are peculiar to the secondary strata, occurring in the various members of the series from the lower lias to the chalk. but having their greatest development in the "as and oolitc. More than 30 species have been discovered; they differ from each other chiefly in the form of the head, some having a long and slender snout, like the gavial of the Gauges, while others had short and broad heads, mor‘3 like the common crocodile.
The great repository for ichthyosaurian remains hitherto has been the lias st Lyme Regis.