TEETH OF ICHTHYOSAURUS CHARACTERISTIC. 233

less from that of the crocodile; the only variation being that the alveoli (which in the crocodile are separate) here run together into one long continuous furrow; in which, indeed the rudiments of a separation into distinct alveoli may be traced, in the slight ridges extending between the teeth along the sides and bottom of the furrow. The appearance and progress of the new tooth, which is to replace the old one, is very nearly the same in the ichthyosaurus and crocodile. In the latter animal they always remain hollow. The new tooth first appears as a germ within the root of the old one, whence by elongation, it rises up into its cavity, and by its increase splits the old tooth, whose fragments soon fall off. In man and most mammalia there is only a single change of teeth; but in the crocodile, there is a continual succession of fresh teeth, the interior is never filled up with bony

matter, and the process may be traced in all its stages in the jaws of crocodiles of every age. In the ichthyosaurus, however, the interior cavity of the teeth is gradually obliterated in old age, by the ossification of the pulpy nucleus. In this animal, therefore, the succession of new sets of teeth is but seldom repeated, and perhaps not more than This curious teething process is represented in the figure of the natural size on the margin.

once.

Osteology of the head of the Ichthyosaurus. The eye is placed rather lower than in the crocodiles.

What is most striking in the head is the enormous size of the eye, and the circle of bony pieces which strengthen the sclerotic in front. These pieces form, as is known, a character common to birds, tortoises, and lizards, to the exclusion of crocodiles and fishes. In fact, the sclerotic of the crocodile is simply cartilaginous; in fishes, it is often osseous in whole or in part, but in them it is never furnished in front with a ring of bony pieces as in birds. This marked character, which had at first, nobody knows why, caused this animal to be likened unto fishes, ought rather to have associated it with lizards. The number of the vertebræ is very considerable. Mr. Conybeare reckons between 80 and 90 of them: M. Cuvier possesses an individual that must have in the fine skeleton of Sir Everard Home, amount had 95. Those distinguishable to 72 at least. As much as the ichthyosaurus resembles lizards by the osteology of its head, so much does it differ in the forms of its vertebræ; and in this respect it approaches at once to fishes and cetacea, as Sir Everard had very justly remarked.

FORM AND HABITS OF THE ANIMAL.

235

The ribs are very slender for so great an animal, and are not compressed, but rather triangular. The shoulder and the breast bone of the ichthyosaurus are disposed essentially as in the lizards. The fin is formed by a series of small bones, comparable to the phalanges of the dolphin, but still more numerous and condensed. All these bones, upwards of 100 in one paddle, are flat, and their angles are tesselated like a pavement, having little motion on each other, and exhibiting no external division in the living animal.

Thus we possess the skeleton of the ichthyosaurus in all its parts, and excepting the form of its scales and the shades of its colours, nothing hinders us from making a complete representation of this animal.

It was a reptile with a moderate tail, and a long pointed muzzle armed with sharp pointed teeth. Two eyes of enormous magnitude must have given its head a most extraordinary aspect, and have facilitated its vision during the night. There was probably no external ear, and the skin passed over the drum bone, as in the cameleon, the salamander, or the pipa, without even becoming any thinner. It naturally breathed air, and not water like fishes; hence it was often obliged to come to the surface. Nevertheless its short, flat, undivided limbs allowed it merely to swim, and it is very probable that it could not even crawl on the shore as well as the seals do; but that if it had the misfortune to be wrecked there, it would remain motionless like whales and dolphins. It lived in a sea inhabited at the same time by mollusca, which have left

ammonites, animals to all appearance, forming species of sepia or pulps, which contained in their interior (as at the present day the nautilus spirula) those spiral and singularly chambered shells. Terebratula with different species of oysters abounded also in this sea, and several sorts of crocodiles frequented its shores; if even they did not inhabit it conjointly with the ichthyosauri.*

We can assign with precision, at least in the species with a slender muzzle (I. tenuirostris) the proportions of its parts. In a total length of three feet and a half, which is that of M. Cuvier's little skeleton, the head and tail take up each one foot, and there remains a foot and a half for the trunk, at the two extremities of which are the fins; for one can scarcely say that there has been a neck. The fore fin (counting the humerus or shoulder bone) was seven inches and a half long, with a breadth of nearly three inches. The posterior fin was a little less both in length and breadth.

The great head of the I. Communis, possessed by M. Cuvier, must have had a length of at least 2 feet and a half; hence it indicates an individual about 9 feet long. A skeleton discovered on the coast of Dorsetshire by Miss Mary Anning has however been referred to this species, although it is only 5 feet long. In reality, among reptiles, the size may vary from double to single, without the teeth denoting the age. But there are much greater ichthyosauri, particularly in the species-platyodon (broad toothed). Miss Anning, it is said, has also discov

⚫ Cuvier. Ossemens Fossiles, Tom. V. Part II. p. 473.

PLESIOSAURUS, or serpent CROCODILE. 237

ered a skeleton 20 feet long. Mr. Johnson possesses a cranium whose breadth behind is two feet six inches, and the longitudinal diameter fourteen inches; and M. Cuvier has vertebræ 6 inches in diameter, which compared to those of his little skeleton whose diameter is only one inch, may have belonged to individuals of twenty-one feet. Cuvier received from Dr. Davis of Bath, a drawing of one of these vertebræ found in oolite near that city, which is nearly 7 inches long. And he has himself fragments of fins from Newcastle which correspond to individuals of a very great size. This species of ichthyosaurus did not come far short of the mosasaurus of Maestricht, whose length has been calculated at 25 feet.

OF THE PLESIOSAURUS.

This genus is also entirely English, and solely due to the sagacity of Mr. Conybeare. Some vertebræ mixed with those of the crocodile and ichthyosaurus, in the lias of the environs of Bristol, appeared to him to differ from those of both animals. A considerable portion of a skeleton in the collection of Colonel Birch, confirmed him in his ideas about the species from which these relics came. In order to complete his arrangement, he added some bones of the extremities found along with these vertebræ, and thus he was enabled to