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]
These two genera are found everywhere in the lias: they were discovered in England, where this rock is exposed in cliffs of great extent; but they have also been found since in France and Germany.
Along with these had lived two species of Crocodiles, the bones of which are also found deposited in the lias, among ammonites, terebratulæ, and other shells of that ancient sea. We have skeletons of them in our cliffs at Honfleur, where the remains are found, from which I have drawn up their characters.[252]
One of these species, the Long-beaked Gavial, has the muzzle longer, and the head more narrow, than the gavial or long-beaked crocodile of the Ganges; the bodies of its vertebræ are convex before, while in our crocodiles of the present day they are so behind. It has been found in the lias deposits of Franconia, as well as in those of France.