The _ichthyosaurus_[249], discovered by Sir Everard Home, has the head of a lizard, but prolonged into an attenuated muzzle, armed with conical and pointed teeth; enormous eyes, the sclerotica of which is strengthened by a frame consisting of bony pieces; a spine composed of flat vertebræ, of a depressed circular form, and concave on both surfaces like those of fishes; slender ribs; a sternum and clavicles like those of lizards and ornithorynchi; a small and weak pelvis; and four limbs, of which the humeri and femurs are short and thick, while the other bones are flattened, and closely set like the stones in a pavement, so as to form, when enveloped with the skin, fins of a single piece, almost incapable of bending; analogous, in short, both as to use and organization, to those of cetacea. These reptiles have lived in the sea; on shore, they could only at most have crept in the hobbling manner of seals; at the same time after they have respired elastic air. The remains of four species have been found: The most extensively distributed (_I. communis_) has blunt conical teeth; its length sometimes exceeds twenty feet. The second (_I. platyodon_), which is at least as large as the former, has compressed teeth, with round and bulging roots. The third (_I. tenuirostris_), has slender and pointed teeth, and the muzzle thin and elongated. The fourth (_I. intermedius_), is, as its name implies, intermediate between the last species and the common, with respect to the form of its teeth. The two latter species do not attain half the size of the two first. The _plesiosaurus_, discovered by Mr Conybeare, must have appeared still more monstrous than the ichthyosaurus. It had the same limbs, but somewhat more elongated and more flexible; its shoulder and pelvis were more robust; its vertebræ had more of the forms and articulations of the lizards; but what distinguished it from all oviparous and viviparous quadrupeds, was a slender neck as long as its body, composed of thirty and odd vertebræ, a number greater than that of the neck of any other animal, rising from the trunk like the body of a serpent, and terminating in a very small head, in which all the essential characters of that of the lizard family are observed. If any thing could justify those hydras and other monsters, the figures of which are so often presented in the monuments of the middle ages, it would incontestibly be this plesiosaurus.[250] Five species are already known, of which the most generally distributed (_P. dolichodeirus_) attains a length of more than twenty feet. A second species (_P. recentior_), found in more modern strata, has the vertebræ flatter. A third (_P. carinatus_) shews a ridge on the under surface of its vertebræ. A fourth, and lastly a fifth (_P. pentagonus_ and _P. trigonus_), have the ribs marked with five and three ridges.[251]