A powerful tail, whose flukes were set vertical, and the lower one of which was vertebrated, served as propeller, while a large dorsal fin was developed as a cutwater. The primitive biconcave vertebræ of the fish and of the early land vertebrates were retained, and the limbs degenerated into short paddles. The skin of the ichthyosaur was smooth like that of a whale, and its food was largely fish and cephalopods, as the fossil contents of its stomach prove. These sea monsters disported along the Pacific shore over northern California in Triassic times, and the bones of immense members of the family occur in the Jurassic strata of Wyoming. Like whales and seals, the ichthyosaurs were descended from land vertebrates which had become adapted to a marine habitat. [Illustration: Fig. 334. Plesiosaurus] _Plesiosaurs_ were another order which ranged throughout the Mesozoic. Descended from small amphibious animals, they later included great marine reptiles, characterized in the typical genus by long neck, snakelike head, and immense paddles. They swam in the Cretaceous interior sea of western North America. [Illustration: Fig. 335. Restoration of a Mosasaur] _Mosasaurs_ belong to the same order as do snakes and lizards, and are an offshoot of the same ancestral line of land reptiles. These snakelike creatures--which measured as much as forty-five feet in length--abounded in the Cretaceous seas. They had large conical teeth, and their limbs had become stout paddles. The lower jaw of the mosasaur was jointed; the quadrate bone, which in all reptiles connects the bone of the lower jaw with the skull, was movable, and as in snakes the lower jaw could be used in thrusting prey down the throat. The family became extinct at the end of the Mesozoic, and left no descendants. One may imitate the movement of the lower jaw of the mosasaur by extending the arms, clasping the hands, and bending the elbows. =Flying reptiles.= The atmosphere, which had hitherto been tenanted only by insects, was first conquered by the vertebrates in the Mesozoic. _Pterosaurs_, winged reptiles, whose whole organism was adapted for flight through the air, appeared in the Jurassic and passed off the stage of existence before the end of the Cretaceous. The bones were hollow, as are those of birds. The sternum, or breastbone, was given a keel for the attachment of the wing muscles. The fifth finger, prodigiously lengthened, was turned backward to support a membrane which was attached to the body and extended to the base of the tail. The other fingers were free, and armed with sharp and delicate claws, as shown in Figures 336 and 337. [Illustration: Fig. 336. Restoration of a Pterosaur] [Illustration: Fig. 337. Skeletons of Pterosaur Ornithostoma, _A_, and of the Condor, _B_ After Lucas]