“Lias” is an English provincial name adopted in geology, and applied to a formation of limestone, marl, and petrified clay, which forms the base of the oolite, or immediately underlies that division of secondary rocks. The lias has been traced throughout a great part of Europe, forming beds of a thickness varying from 500 to 1000 feet of the above-mentioned substances, which have been gradually deposited from a sea of corresponding extent and direction. The lias abounds with marine shells of extinct species, and with remains of fishes that were clad with large and hard shining scales. Of the higher or air-breathing animals of that period, the most characteristic were the

Enaliosauria.

The creatures called Enaliosauria or Sea-lizards (from the Greek enalios, of the sea, and sauros, lizard), were vertebrate animals, or had back bones, breathed the air like land quadrupeds, but were cold-blooded, or of a low temperature, like crocodiles and other reptiles. The proof that the Enaliosaurs respired atmospheric air immediately, and did not breathe water by means of gills like fishes, is afforded by the absence of the bony framework of the gill apparatus, and by the presence, position, and structure of the air passages leading from the nostrils, and also by the bony mechanism of the capacious chest or thoracic-abdominal cavity: all of which characters have been demonstrated by their fossil skeletons. With these characters the Sea-lizards combined the presence of two pairs of limbs shaped like fins, and adapted for swimming.

The Enaliosauria offer two principal modifications of their anatomical, and especially their bony, structure, of which the two kinds grouped together under the respective names of Ichthyosaurus and Plesiosaurus are the examples.

The Ichthyosaurus.

The genus Ichthyosaurus includes many species: of which three of the best known and most remarkable have been selected for restoration to illustrate this most singular of the extinct forms of animal life.

The name (from the Greek ichthys, a fish, and sauros, a lizard) indicates the closer affinity of the Ichthyosaur, as compared with the Plesiosaur, to the class of fishes. The Ichthyosaurs are remarkable for the shortness of the neck and the equality of the width of the back of the head with the front of the chest, impressing the observer of the fossil skeleton with a conviction that the ancient animal must have resembled the whale tribe and the fishes in the absence of any intervening constriction or “neck.”

This close approximation in the Ichthyosaurs to the form of the most strictly aquatic back-boned (vertebrate) animals of the existing creation is accompanied by an important modification of the surfaces forming the joints of the back-bone, each of which surfaces is hollow, leading to the inference that they were originally connected together by an elastic bag, or “capsule,” filled with fluid—a structure which prevails in the class of fishes, but not in any of the whale or porpoise tribe, nor in any, save a few of the very lowest and most fish-like, of the existing reptiles.

With the above modifications of the head, trunk, and limbs, in relation to swimming, there co-exist corresponding modifications of the tail. The bones of this part are much more numerous than in the Plesiosaurs, and the entire tail is consequently longer; but it does not show any of those modifications that characterise the bony support of the tail in fishes. The numerous “caudal vertebrÆ” of the Ichthyosaurus gradually decrease in size to the end of the tail, where they assume a compressed form, or are flattened from side to side, and thus the tail instead of being short and broad, as in fishes, is lengthened out as in crocodiles.

The very frequent occurrence of a fracture of the tail, about one fourth of the way from its extremity, in well-preserved and entire fossil skeletons, is owing to that proportion of the end of the tail having supported a tail-fin. The only evidence which the fossil skeleton of a whale would yield of the powerful horizontal tail-fin characteristic of the living animal, is the depressed or horizontally flattened form of the bones supporting such fin. It is inferred, therefore, from the corresponding bones of the Ichthyosaurus being flattened from side to side, that it possessed a tegumentary tail-fin expanded in the vertical direction. The shape of a fin composed of such perishable material is of course conjectural, but from analogies, not necessary here to further enlarge upon, it was probably like, or nearly like, that which the able artist engaged in the restoration of the entire form of the animal has given to it. Thus, in the construction of the principal swimming-organ of the Ichthyosaurus we may trace, as in other parts of its structure, a combination of mammalian (beast-like), saurian (lizard-like), and piscine (fish-like) peculiarities. In its great length and gradual diminution we perceive its saurian character; the tegumentary nature of the fin, unsustained by bony fin-rays, bespeaks its affinity to the same part in the mammalian whales and porpoises; whilst its vertical position makes it closely resemble the tail-fin of the fish.

The horizontality of the tail-fin of the whale tribe is essentially connected with their necessities as warm-blooded animals breathing atmospheric air; without this means of displacing a mass of water in the vertical direction, the head of the whale could not be brought with the required rapidity to the surface to respire; but the Ichthyosaurs, not being warm-blooded, or quick breathers, would not need to bring their head to the surface so frequently, or so rapidly, as the whale; and, moreover, a compensation for the want of horizontality of their tail-fin was provided by the addition of a pair of hind-paddles, which are not present in the whale tribe. The vertical fin was a more efficient organ in the rapid cleaving of the liquid element, when the Ichthyosaurs were in pursuit of their prey, or escaping from an enemy.

That the Ichthyosaurs occasionally sought the shores, crawled on the strand, and basked in the sunshine, may be inferred from the bony structure connected with their fore-fins, which does not exist in any porpoise, dolphin, grampus, or whale; and for want of which, chiefly, those warm-blooded, air-breathing, marine animals are so helpless when left high and dry on the sands: the structure in question in the Ichthyosaur is a strong osseous arch, inverted and spanning across beneath the chest from one shoulder-joint to the other; and what is most remarkable in the structure of this “scapular” arch, as it is called, is, that it closely resembles, in the number, shape, and disposition of its bones, the same part in the singular aquatic mammalian quadruped of Australia, called Ornithorhynchus, Platypus, and Duck-mole. The Ichthyosaurs, when so visiting the shore, either for sleep, or procreation, would lie, or crawl prostrate, or with the belly resting or dragging on the ground.

The most extraordinary feature of the head was the enormous magnitude of the eye; and from the quantity of light admitted by the expanded pupil it must have possessed great powers of vision, especially in the dusk. It is not uncommon to find in front of the orbit (cavity for the eye), in fossil skulls, a circular series of petrified thin bony plates, ranged round a central aperture, where the pupil of the eye was placed. The eyes of many fishes are defended by a bony covering consisting of two pieces; but a compound circle of overlapping plates is now found only in the eyes of turtles, tortoises, lizards, and birds. This curious apparatus of bony plates would aid in protecting the eyeball from the waves of the sea when the Ichthyosaurus rose to the surface, and from the pressure of the dense element when it dived to great depths; and they show, writes Dr. Buckland,[4] “that the enormous eye, of which they formed the front, was an optical instrument of varied and prodigious power, enabling the Ichthyosaurus to descry its prey at great or little distances, in the obscurity of night, and in the depths of the sea.”

Of no extinct reptile are the materials for a complete and exact restoration more abundant and satisfactory than of the Ichthyosaurus; they plainly show that its general external figure must have been that of a huge predatory abdominal fish, with a longer tail, and a smaller tail-fin: scale-less, moreover, and covered by a smooth, or finely wrinkled skin analogous to that of the whale tribe.

The mouth was wide, and the jaws long, and armed with numerous pointed teeth, indicative of a predatory and carnivorous nature in all the species; but these differed from one another in regard to the relative strength of the jaws, and the relative size and length of the teeth.

Masses of masticated bones and scales of extinct fishes, that lived in the same seas and at the same period as the Ichthyosaurus, have been found under the ribs of fossil specimens, in the situation where the stomach of the animal was placed; smaller, harder, and more digested masses, containing also fish-bones and scales have been found, bearing the impression of the structure of the internal surface of the intestine of the great predatory sea-lizard. These digested masses are called “coprolites.”

In tracing the evidences of creative power from the earlier to the later formations of the earth’s crust, remains of the Ichthyosaurus are first found in the lower lias, and occur, more or less abundantly, through all the superincumbent secondary strata up to, and inclusive of, the chalk formations. They are most numerous in the lias and oolite, and the largest and most characteristic species have been found in these formations.

No. 12.—Ichthyosaurus platyodon.

This most gigantic species, so called on account of the crown of the tooth being more flattened than in other species, and having sharp edges, as well as a sharp point, was first discovered in the lias of Lyme Regis, in Dorsetshire. Fossil remains now in the British Museum, and in the museum of the Geological Society, fully bear out the dimensions exhibited by the restoration of the animal as seen basking on the shore between the two specimens of Long-necked Plesiosaurs. The head of this species is relatively larger in proportion to the trunk, than in the Ichthyosaurus communis or Ichthyosaurus tenuirostris: the lower jaw is remarkably massive and powerful, and projects backwards beyond the joint, as far as it does in the crocodile. In the skull of an individual of this species, preserved in the apartments of the Geological Society of London, the cavity for the eye, or orbit, measures, in its long diameter, fourteen inches. The fore and hind paddles are large and of equal size.

The lias of the valley of Lyme Regis, Dorsetshire, is the chief grave-yard of the Ichthyosaurus platyodon; but its remains are pretty widely distributed. They have been found in the lias of Glastonbury, of Bristol, of Scarborough and Whitby, and of Bitton, in Gloucestershire; some vertebrÆ, apparently of this species, have likewise been found in the lias at Ohmden, in Germany.