Owing to the comparative abundance and excellent preservation of ichthy.>saurian remains, the hard parts of those creatures have been studied under exceptionally favourable circumstances, and much has thus been learnt of their structure and, by inference therefrom, of their life history. They were large marine reptiles, measuring in some instances 30 feet in length, and somewhat resembling in appearance the dolphins of the present day. Like these they were air-breathers, and must therefore have come to the surface to breathe, although being coldblooded they were no doubt able, like the aquatic saurians of our own period, to remain much longer under water than the warm-blooded Cetacea. The ichthyosaurian head was large, and was prolonged into a more or less elongated snout, certain species rivalling in this re pect the gavial of the Ganges. The brain cavity, on the other hand, was remarkably small. The eyes were enormously large, the orbit in Ichthyosaurusplatyodon— the largest known species —having been found to measure 14 inches in long diameter. This huge eyeball was protected by a ring of bony sclerotic plates similar to those found in rapacious birds and in turtles and lizards of the present day. The jaws, which in Ichthyosaurus platyodon have been known to measure 6 feet in length, were rendered still more formidable by their array of strong, conical, pointed teeth, numbering in some instances over one hundred and eighty, and placed not in distinct sockets, as in the crocodile, but in a common alveolar groove. These, as they became worn, were replaced by a succession of young teeth, which budded up at the base of the old. The neck in the ichthyosaurus was extremely short, and not marked by any constriction. The vertebrae resembled those of fishes in being deeply biconcave. The tail was long and tapering; in many specimens this organ has been found to be fractured at about a fourth of its length from the extremity, and, as the vertebrae of the same region seem to have been flattened vertically, Professor Owen regards it as probable that these reptiles were provided with a tegumentary caudal fin like that of the Cetacea, only vertical instead of horizontal; the sole evidence of the presence of such a horizontal fin in extinct whales would be the horizontally flattened condition of the last caudal vertebrae, should any of these chance to be preserved. Ichthyosaurians were provided with two pairs of limbs in the form of paddles, which externally must have borne considerable resemblance to the anterior limbs of dolphins and other Cetacea. They differed, however, very markedly from these in the possession of a bony apparatus, stretching, in the case of the front pair, from one shoulder joint to the other, on which the anterior paddles were supported. This “scapular arch,” according to Professor Owen, resembled, “in the number, shape, and disposition of its bones,” the same parts in the Australian Orwithorhynchus, a mammal which leads, as Ichthyosaurus did, an aquatic life, obtaining its food at the bottom of lakes and rivers, but having to rise frequently to the surface to breathe. The hind limbs were in almost all the species much smaller than the pair in front. The skeleton of each of the paddles consisted mainly of a large number —in some species exceeding a hundred—of small polygonal bones arranged in more than five closely packed longitudinal rows, the whole covered with skin, and forming a highly elastic organ of locomotion. The ichthyosaur was provided with slender ribs along the vertebral column from the anterior part of the neck to the tail; a sternum, however, was wanting, the abdominal walls being strengthened by the development of transverse arcuated bones. As no trace of horny scales or bony scutes has ever been detected in connexion with those reptilian remains, it may be assumed that these sea-saurians, as they have been called, were, like the Cetacea of the present day, covered with a smooth or wrinkled skin unprovided with any of those dermal appendages.

From a study of their bony structure it may be inferred that these huge aquatic reptiles inhabited the open sea, occasionally visiting the shores, where their powerful paddles enabled them to crawl on land, and where, like seals, they probably loved to bask in the sunshine. That they were predatory in their habits—the tyrants indeed of Mesozoic seas—might be inftr.ed from our knowledge of their jaws and teeth; and this is amply confirmed by an examination of the half-digested contents of their stomachs.

Their food seems to have consisted chiefly of ganoid fishes and the smaller reptiles, and as the vertebrae and other remains of young ichthyosaurs have occasionally been found mixed with these, there is reason to believe that they, like many other marine animals, did not hesitate to devour the weaker members of their own species. In several instances tolerably complete skeletons of small ichthyosaurs have thus been found enclosed within the ribs of larger individuals of the same species, and their occurrence gave rise to the evidently erroneous conjecture that those reptiles might have been viviparous. The fact that the entombed specimens have in almost every case been found with the head turned towards the tail of the enclosing animal was supposed to favour this view; the discovery, however, of additional specimens may at any time deprive this argument of the little value it has, and recently Professor Merian described a specimen from the Upper Lias of Würtemberg in which the included ichthyosaur lay with its head towards that of the enveloping specimen. The nature of their food is indicated, not only by the occurrence, in what from its position must have been the stomach, of the half-digested remains of fishes and reptiles, but also by the presence of similar relics, and especially of the scales of fishes, in their faeces. The coprolites of the ichthyosaurians are oval bodies measuring usually from 2 to 4 inches in length, and exhibiting on their surface the impression of the spirally convoluted internal surface of the intestine. These coprolites consist chiefly of phosphate of lime, and occur in great abundance in certain Liassic beds, where, says Buckland, they look “like potatoes scattered on the ground.”

The species of the genus Ichthyosaurus differ from each other chiefly in the proportion of certain parts of the body and of the teeth. Professor Huxley has divided them into two groups :—(1) those which have relatively short snouts and short paddles, with four carpalia, including therein such forms as I. intermedius and I. communis, the latter remarkable as having its anterior paddles three times the length of the pair behind; and (2) those with longer snouts, long paddles, and three carpalia, including such forms as I. longirostris and I, tenuirostris, which in the length of their snouts rival the gavial of the Ganges, and I. platyodon, in which the fore and hind limbs are of equal length.