Many of the characteristics of the limbs of the plesiosaurs are peculiar to themselves; others they had in common with other aquatic reptiles and mammals. The paddles resemble those of the ichthyosaurs more nearly than those of any other reptile, and it was doubtless this superficial resemblance which so long deceived the early anatomists as to the affinities of the two orders. Unlike all other aquatic animals, however, the plesiosaurs have the hind limbs nearly or quite as large as the front ones, and they doubtless were equally effective in function. The humerus and femur are always elongate, though broad and massive. In no other aquatic animals, save the marine turtles, do we find these bones relatively so long and strong; they are very short in the cetaceans, the sirenians, the ichthyosaurs, mosasaurs, thalattosaurs, and the marine crocodiles, in front at least. The strong muscular rugosities of the plesiosaurian bones are very suggestive of powerful swimming muscles. [Illustration: FIG. 37.—Pelvic girdle of _Elasmosaurus_: _p_, pubis; _is_, ischium; _il_, ilium.] [Illustration: FIG. 38.—Paddles of Plesiosaurs: _A_, right hind paddle of _Thaumatosaurus_, after Fraas; _B_, right hind paddle of _Trinacromerum_; _C_, right front paddle of same individual; _f_, femur; _fb_, fibula; _t_, tibia; _h_, humerus; _r_, radius; _u_, ulna.] The bones of the forearms and legs, the wrists and ankles are all polygonal platelets of bones, closely articulating with each other. The finger and toe bones have a more elongated, hour-glass shape than those of the ichthyosaurs, resembling more nearly those of the mosasaurs, indicating a greater flexibility than the ichthyosaurs possessed. The ichthyosaur paddles must have been quite like the fins of fishes in function, while doubtless those of the plesiosaurs were capable of a more varied use, as indeed was required of them. Their articulation with the trunk was more of a ball-and-socket joint than in the other reptiles, showing possibility of considerable rotation on the long axis, and an antero-posterior propelling action. The paddles were certainly more powerful than those of any other aquatic air-breathing animals. There were no additional digits, all plesiosaurs having neither more nor less than five in each hand and foot. Hyperphalangy was sometimes carried to an excessive degree, some digits of some species having as many as twenty-four bones, a larger number than has been observed in any other air-breathing vertebrate. [Illustration: FIG. 39.—Pectoral girdle (in part) and front paddles of _Elasmosaurus_ (after Riggs): _sc_, scapula; _h_, humerus; _cor_, coracoid; _r_, radius; _u_, ulna.]