Ichthyosaurus, the fish-lizard, the only genus of an extinct order of reptiles, the Ichthyopterygia. More than thirty species have been described from the Secondary rocks of the Old World, especial] y from the Lias, and, very perfect specimens of both young and adult forms having been obtained, their anatomy is very completely known. They are sometimes thirty feet long, and somewhat resemble the dolphin in general form. The head is large, having a long, gavial-like snout and very large orbits; but a very small brain cavity. The jaws, sometimes six feet, long, may contain over 180 teeth, which are conical, and are not in distinct sockets, as in the crocodile, but in a common alveolar groove, and can be replaced from below. The eyes are surrounded by bony sclerotic plates, like those in turtles, owls, etc., the whole orbit sometimes reaching fourteen inches in diameter. The neck is so short as to be probably invisible externally, and the numerous vertebree are deeply biconcave, like those of fishes. The tail is long and tapering; and from its extremity being generally found in a dislocated condition, Owen assumed a large vertical caudal fin, the presence of which has been recently demonstrated in exceptionally perfect specimens. .The limbs are short, the hind ones being the smaller, and they terminate in paddles with rows of marginal ossicles in addition to the five usual digits. There are numerous slender ribs, but no breast-bone. Traces of the skin have been found, and show no sign of scales or bony plates. The discovery of half-digested remains of fish, reptiles, and young ichthyosaurs within their ribs and in their spirally-convoluted coprolites (q.v.), which are two to four inches long, point to the predatory habits of the animals. They may have crawled on land, like seals; but were adapted for deep water, being better able to remain under water than warm-blooded animals such as whales or seals.