As in all other aquatic reptiles, it is in the limbs that the most striking characteristics of these water lizards or “sea-serpents” are found. The legs were so completely adapted to an aquatic mode of living that the animals must have been practically helpless upon land, able perhaps to move about in a serpentine way when accidentally stranded upon the beaches, but probably never seeking the land voluntarily. The front limbs, like those of all other swimming animals having a powerful propelling tail, were larger than the hind ones, though not very much so. The bones of the first two segments, that is, the arm, forearm, and thigh and leg bones, were all short and broad, resembling those of the ichthyosaurs more than those of any other reptiles, save perhaps the thalattosaurs, discussed below. The articular surfaces of all the limb bones, as in other aquatic animals, were restricted in extent, indicating limited motion between the joints, though doubtless having great flexibility. In the most specialized types, such as _Tylosaurus_, the wrist and ankle bones were almost wholly cartilaginous, just as they are in the water salamanders, and in whales and porpoises. This tendency of the ends of long bones, the wrists and ankles as well as other bones of the skeleton, to become more cartilaginous, or less well ossified, in animals purely aquatic in habit is a marked one. So much is this the case that paleontologists always suspect water habits in reptiles showing it, even though but few parts of the skeleton are known. [Illustration: FIG. 74.—_Clidastes_; left front paddle: _c_, coracoid; _h_, humerus; _r_, radius; _sc_, scapula; _u_, ulna.] [Illustration: FIG. 75.—_Tylosaurus_; left front paddle: _c_, coracoid; _sc_, scapula; _h_, humerus; _r_, radius; _u_, ulna.]