We have seen that many skeletons of ichthyosaurs are found entire, and but little disturbed in position, suggesting, if not proving, that the animals as a rule lived and died far out in the deep seas, away from the disturbing effects of currents of water on their decaying bodies. Among the thousands collected, the great majority of the specimens of mosasaurs consist of a few bones or a part of the skeleton only. Moreover, nearly all specimens show the disturbing effects of currents of water; and the bones are usually associated with those of turtles, birds, and flying reptiles, which probably did not often venture far from the shores; all of which goes to prove that the mosasaurs in general lived in the comparatively shallow waters of the seas, and not far from the shores. That some were excellent divers, descending probably many fathoms deep in the water, is certain, because of the extraordinary protective structures of the eyes and ears. But the various kinds of mosasaurs differed not a little in their habits. Some, like _Mosasaurus_ and _Clidastes_, were doubtless chiefly surface swimmers, as is evidenced by their better ossified bones, firmer articulation, and the presence of the additional zygosphenal articulations of the vertebrae, wanting in other forms, as also by the structure of their paddles. They had a relatively long body and short tail, the tail having a more pronounced distal expansion than in the case of other forms, and the eyes looking laterally, not at all upward. The feet, as shown in Fig. 74, were broad and short, with most of the wrist and ankle bones well ossified, and with but few extra bones in the digits. _Tylosaurus_ (Fig. 79), on the other hand, had a more slender skull, the nostrils were situated farther back from the tip of the snout, the tail was longer and more powerful, and the feet were very highly specialized (Fig. 75). The wrist and the ankle were almost wholly cartilaginous, the fifth finger and fifth toe were much longer, and the number of phalanges was greatly increased. Moreover, the bones of the skeleton are more spongy, the joints are more cartilaginous, and the ears were better protected by a heavy coat of cartilage. In most of these respects the genus _Platecarpus_ was intermediate between _Clidastes_ and _Tylosaurus_ (Fig. 76). Like nearly all other lizards, the mosasaurs had a pineal opening in the skull, but it is not at all probable that they possessed a functional pineal eye.