ICHTHYOSAURUS  , a

fish or porpoise-shaped marine reptile which characterized the Mesozoic period and became extinct immediately after the deposition of the Chalk . It was named Ichthyosaurus (Gr. fish-lizard) by C . Konig in 1818 in allusion to its outward form, and is best known by nearly complete skeletons from the Lias of England and Germany . The large head is produced into a slender, pointed snout; and the jaws are provided with a row of conical teeth nearly uniform in size and deeply implanted in a continuous groove . The eye is enormous, and is surrounded by a ring of overlapping " sclerotic From British Museum Guide to Fossil Reptiles and Fishes, by permission of the Trustees . Skeleton of Ichthyosaurus communis, with outline of body and fins, from the Lower Lias of Lyme Regis, Dorset; original nearly four metres in length . plates," which would serve to protect the eye-ball during diving . The vertebrae are very numerous, short and deeply biconcave,. imparting great flexibility to the backbone as in fishes . The neck is so short and thick that it is practically absent . There are always two pairs of paddle-like limbs, the hinder pair never disappearing as in porpoises and other Cetacea, though often much reduced in size . A few specimens from the Upper Lias of Wurttemberg (in the museums of Stuttgart, Tubingen, Budapest and Chicago) exhibit remains of the skin, which is quite smooth and forms two triangular median fins, one in the middle of the back, the other at the end of the tail . The dorsal fin consists merely of skin without any internal skeleton, while the tail-fin is expanded in a vertical plane and has the lower lobe stiffened by the tapering end of the backbone, which is sharply bent downwards .

Immature individuals are sometimes observable within the full-grown skeletons, suggesting that this reptile was viviparous . The largest known

species of Ichthyosaurus is I. trigonodon from the Upper Lias of Banz, Bavaria, with the head measuring about two metres in length and probably representing an animal not less than ten metres in total length . I. platyodon, from the English Lower Lias, seems to have been almost equally large . I. intermedius and I. communis, which are the commonest species in the English Lower Lias, rarely exceed a length of three or four metres . The species in rocks later than the Lias are known for the most part only by fragments, but the remains of Lower Cretaceous age are noteworthy for their very wide geographical distribution, having been found in Europe, the East Indies . Australia, New Zealand and South America . Allied Ichthyosaurians named Ophthalmosaurus and Baptanodon, from the Upper Jurassic of England and North America, are nearly or quite toothless and have very flexible broad paddles . The earliest known Ichthyosaurians (Mixosaurus), which occur in the Trias, are of diminutive size, with paddles which suggest that these marine reptiles were originally descended from land or marsh animals (see REPTILES) .