A little higher is the limestone called Alpine, and resting upon it the shell-limestone, so rich in entrochites and encrinites, which forms the basis of a great part of Germany and Lorraine.
In it have been found skeletons of a very large sea-tortoise, the shells of which might have been from six to eight feet in length; and those of another oviparous quadruped of the family of lizards, of a large size, and with a very sharp muzzle[248].
Rising still through sandstones, which present only vegetable impressions of large arundinaceæ, bamboos, palms, and other monocotyledonous plants, we come to the different strata of the deposit which has been named the Jura limestone, on account of its forming the principal nucleus of that chain of mountains.
It is here that the class of Reptiles assumes its full development, and shews itself under the most varied forms and gigantic sizes.
The middle part, which is composed of oolites and lias, or of grey sandstone containing gryphites, contains the remains of two genera, the most extraordinary of all, which have combined the characters of the class of oviparous quadrupeds with organs of motion similar to those of the cetacea.
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.