Ichthyosaurus (Gr. fish, and lizard), a gigantic fossil marine reptile, belonging to the order enaliosaurians of Conybeare. The body was fish-like in form, with a large head, neck of equal width with occiput and thorax; the vertebra) had biconcave articular surfaces, as in fishes and the perennibranchiate reptiles; the paddles, four in number, were comparatively small, resembling in form those of cetaceans, but in the number of digits and of their constituent bones and appended bifurcated rays they came near the structure of the fins of fishes; the tail was long, the vertebrae gradually becoming smaller and flatter toward the end, and probably margined with a tegu-mentary fin expanded or in a vertical direction ; the tail was doubtless the principal organ of locomotion, and presented the saurian character of length and gradual diminution, being cetacean in its partially tegumentary nature, and fish-like in its vertical position. According to Dr. Buckland, the skin was scaleless and finely wrinkled, as in cetaceans.
The skull is like that of the dolphin, with a smaller cerebral cavity and an unanchylosed condition of the cranial bones; the intermaxillaries are greatly developed, and the orbits immense, surrounded by numerous large sclerotic plates; in the convex articulating surface of the occiput, the solid structure of the back part of the skull, and the massive proportions of the jaws and the bones with which they are articulated, we see crocodilian affinities. The nostrils are a short distance in front of the orbits; the teeth are situated in an alveolar groove with their bases free, and separated by partial ridges, the roots being implanted much as in the crocodile; hence this reptile is placed by Prof. Agassiz in the order of rhizodonts. The structure of the hyoid apparatus indicates that it was an air breather, with a slightly developed tongue, and that it obtained its food in the water, having an apparatus, as in the crocodile, to shut off the cavity of the mouth from the larynx. The ribs are well developed, extending from near the head to the tail, and attached to a large sternum; the clavicles and shoulder blades are strong; the resulting pectoral arch resembles much that of the mammalian orni-thorhynchus, and is very different from that of the cetaceans, indicating that the anterior limbs were used not only in swimming but in crawling up the shores of the ocean for the purpose of depositing their eggs, etc.
The arm and forearm are very short and broad; after these come the bones of the wrist and fingers, arranged as flattened ossicles in series of from three to six, so dovetailed together at the sides as to form one powerful framework. The pelvic arch is not articulated to the spine, but was merely suspended in the muscles, as in fishes; the posterior limbs or paddles are generally considerably smaller than the anterior, and would seem to have been more serviceable in terrestrial progression than in swimming. The best known species, I. communis (Cony-beare), grew to a length of 20 ft.; the large conical, longitudinally furrowed teeth are from 40 to 50 above on each side, and 25 to 30 below; the jaws are prolonged and compressed, the vertebrae about 140, with the anterior paddles three times as large as the posterior; like all the species, this is found in the secondary formations, principally in the lias and oolite of England. The I. intermedins (Conyb.), the most common and generally distributed of the species, does not much exceed 7 ft. in length; the teeth are more acutely conical, and about 40-35-40-35; the vertebrae are about 130, and the fore paddles are much the larger.
The I. platyodon (Conyb.), so called from the greater smoothness and flatness of the crowns of the teeth, must have attained a length of more than 30 ft.; the head is longer than in the preceding species, and the jaws broader and more powerful; the teeth are about 45/40-45/40, and are frequently found broken as if from its own violence; the vertebrae are about 120; the most remarkable character is the equality in size of the fore and hind paddles, and the comparative simplicity of their structure. The I. tenuirostris (Conyb.) is characterized by the length and slenderness of the jaws, as in the gavial; this, with the flat head and large orbits, gives to the skull, as Owen says, the appearance of that of a gigantic snipe with its bill armed with teeth; the teeth are slender and very numerous, about 70/60-70/60, and directed obliquely backward; it attained a length of about 15 ft., and was rather slender in its proportions. Six other species, and details on all, will be found in Prof. Owen's " Report on British Fossil Reptiles to the British Association," in 1839. Their remains extend through the whole of the oolitic period, including the lias and oolite proper to the Wealden and chalk formations, in Great Britain and central Europe. For fuller details the reader is referred to the writings of Conybeare, Cuvier, and Buckland. These reptiles, of gigantic size and marine habits, must have been very active and destructive; their food, as indicated by the bones and scales found with their remains, consisted principally of fishes.
From the great size of the eyes, they could probably see well by night; being air breathers, like the crocodiles, they no doubt seized their prey near the surface; the immense cuttle fishes of the secondary epoch probably furnished a portion of their food. These strange creatures formed the connecting link between reptiles and fishes, as do the perennibranchiate amphibia in the actual creation; and by some they have been considered, like the latter, as possessors of both gills and lungs, at least in some stage of their existence, and therefore to a certain extent amphibious. This reptile, with the muzzle of a dolphin, the teeth of a crocodile, the head of a lizard, the paddles of a whale, and the vertebrae of a fish, buried for myriads of years, was introduced to the scientific world by Sir Everard Home, in the "Philosophical Transactions " for 1814.
Skeleton of Ichthyosaurus.