Plesiosaurus, an extinct gigantic enaliosau-rian or marine reptile, found principally in the lias (secondary) formation of England, in company with the still larger ichthyosaurus. The head was small, supported on a long, flexible, snake-like neck, the body and tail short, with four limbs in the shape of powerful swimming paddles, like those of turtles or cetaceans; the skin was probably naked. This singular genus, named by Conybeare, to a lizard's head united the teeth of a crocodile, a neck like a serpent's body, the trunk and tail of a quadruped, the vertebra of a fish, the ribs of a chameleon, and the fins of a whale. The apertures through which the air was respired are just in front of the orbits on the highest part of the head, and not at the end of the snout as in crocodiles; the paddles were probably invested with a sheath of integument, and from the natural curvature of the bones must have had a more elegant and tapering form and greater flexibility than in cetaceans. Nearly 20 species are de scribed, of which the best known is the P. dolicliodeirus (Conyb.), which attained a length of 10 to 12 ft.; there were about 50 teeth in each jaw; the neck was as long as the body and tail together, having 33 vertebrae, 10 more than the longest neck of a bird; the ribs were united in front by several cartilages, enabling the animal to inflate the lungs readily and fully, and take in a supply of air for a prolonged immersion; the coracoid bones were very large, producing an elongation of the sternum, and indicating that the animal was aquatic, and able only with difficulty to drag itself along on land.

The less strong and less numerous teeth show a less carnivorous disposition than in the ichthyosaurus; the slighter general conformation was suited rather for tranquil waters than to encounter powerful waves. Species are found also in the oolite and cretaceous strata, though less abundantly than in the lias, in which their numerous remains and coprolites show that the waters must have swarmed with them. For full details on the species, see Owen's " Report on British Fossil Reptiles," in " Reports of the British Association " for 1839. - The genus pliosaurus (Owen) includes the gigantic reptiles of the Oxford and Kim-meridge clays of England, intermediate between the plesiosaurus and the ichthyosaurus; the teeth and the bones of the limbs and trunk were like those of the former, the first being stouter and more trenchant; but they had the short neck, massive head, and cetacean-like form of the latter genus. It is interesting to note that pliosaurus did not appear until after both the genera which it in part resembles; no fragment of their bones, according to Pic-tet, has been found in the lias or oolite, and none until the time of the Oxford clays.

The best known species is the P. brachydeirus (Owen), for an account of which see Owen's report in " Reports of the British Association " for 1841.

Plesiosaurus dolichodeirus (restored).

Skeleton of Plesiosaurus.