PLESIOSAURUS (Gr. near to a lizard), a remarkable genus of fossil sea-reptiles, the species of which are found in the lias, oolite, and cretaceous measures. Its remains ate so abundant and so perfectly preserved, that we are as well acquainted with skeletons of many of its species as we are with those of any living animals. These represent a strange animal, the structure of which envier considers to be the most singular, and its character the most anomalous, that had been discovered 'mid the ruins of former worlds. In the words of Buekland, " To the head of a lizard, it the teeth of a crocodile, a neck of enormous length, resembling the body of a serpent, a trunk and tail having the proportions of an ordinary quadruped, the ribs of a chameleon, and the paddles of a whale." The skull is small and depressed. From the nostrils backward, it is quadrate; it suddenly contracts at the nostrils, and is continued into a parallel-sided apex, which is sometimes slightly swollen at the point. No sclerotic plates have been found m the orbits. The rami of the lower jaw are remarkably expanded at their anterior anchylosed extremity. No intervening vacuity separates the angular and surangular pieces, as in the crocodiles, but they are joined throughout, as in the lizards. The teeth occupy dis tinct cavities; they are sharp-pointed, long, slender, circular in cross-section, and with fine longitudinal ridges on the enamel. The most striking peculiarity of the vertebra is the great length of the neck-portion, which is composed of from 20 to 40 vertebrae. The articular surfaces of the bodies of the vertebra; are either flat or slightly convex in the center, with a concavity round the periphery. The cervical vertebrre consist of a centrum, neural arch, and two ribs, which articulate into two pits on the sides of the centrum. In the dorsal vertebra, the ribs are articulated to diapophyses from the neural arch; and in the tail, they gradually descend again to the sides of the centrum. The tail is much shorter than in the ichthyosaur. In the abdominal region the extremities of each pair of ribs are connected below by the development of the Immal spine.
The two pair of limbs were equal in size and shape, with probably a single exception. The bones of the hind-limbs closely correspond in number, arrangement, and form with those of the fore-limbs, so that the descriptions of the one set answer to the correspond ing bones of the other. • The humerus is a stout and moderately long bone, curved
slightly backwards, rounded at its proximal extremity, and flattened as it approaches the elbow joints. The radius and ulna are short and tiat bones—the former straight, the latter reniform, with the concavity towards the radius. The cal us consists of six to eight flat round bones in a double row. The five metacarpals are long, slender, and slightly expanded at both ends. The numerous phalanges are alike in but gressively decrease in size. The radial digit has generally three; the second from five to seven; the third. eight or nine; the fourth, eight; and the fifth, five or six phalanges. The limbs were covered with integument, so as to form simple undivided paddles, as in the turtle.
The supposed habits of the plesiosaur are thus described by Conybeare: " That it was an aquatic, is evident from the form of its paddles; that it was marine, is almost equally so, from the remains with which it is universally associated; that it may have occasionally visited the shore,. the resemblance of its extremities to those of the turtle may lead us to conjecture; its motion must have, however, beep very awkward on land; its long neck must have impeded its progress through the water, presenting a striking contrast to the organization which so admirably fits the ichthyosaur to cut through the waves. May it not, therefore, bu concluded—since, in addition to these cir cumstances, its respiration must have required frequent access to the air—that it swain upon or near the surface, arching back its long neck like the swan, and occasionally dartittg it down at the fish which happened to float within its reach? It may perhaps have lurked in shoal-water along the coast, concealed among the sea-weed, and, raising its nostrils to the surface from a considerable depth, may have found a secure retreat from the assaults of dangerous enemies; while the length and flexibility of its neck may have compensated for the want of strength in its jaws, and its incapacity for swift motion through the water, by the suddenness and agility of tile attack which they enabled it to make on every animal fitted for its prey which came within its reach." The first remains of this animal were discovered at Lyme Regis hi 1822. Since then 22 species have been described, the specific differences chiefly resting on peculiarities iu the form and structure of the vertebrm.