The external nostrils, of large size, were situated at a considerable distance back of the end of the snout, but not nearly so far back or so near the eyes as were the nostrils of the ichthyosaurs, plesiosaurs, and phytosaurs. Their size and position suggest a use like that of the modern aquatic monitors, as mentioned on a preceding page. The eyes were of moderate size, those of the less purely aquatic forms being directed more laterally than those of species of more distinctly diving habits. They were protected by a stout ring of bony plates, as were the eyes of all truly aquatic reptiles of the past. The ears, also, in most if not all mosasaurs, had a thick cartilaginous ear-drum in place of a simple membrane, evidently, as Dollo has shown, for better protection under undue pressure of the water in deep diving.
Fig. 71.—Clidastes, inner side of right mandible: ang, angular; art, articular; cor, coronoid; pa, prearticular; sur, surangular.
As in all other lizards, the bones with which the lower jaws articulate, the quadrates, were loosely attached at the upper end, permitting great freedom of movement in all directions, more even than the land lizards have. The lower jaws were long and powerful, armed with a single row of teeth on each side, from sixteen to eighteen in number. Just back of the teeth, a little beyond the middle, each mandible has a remarkable joint, quite unknown in land lizards, though a trace of it is found in the monitors, permitting much movement between the front and back parts, both laterally and vertically, though chiefly in the former direction. Furthermore, as in land snakes but not as in land lizards, the front ends of the two sides of the jaws were somewhat loosely attached to each other by ligaments. This looseness of the two sides of the jaws, not only in front but also behind, together with the joint in each, was of the greatest use in swallowing prey, as will be explained farther on.
Fig. 72.—Skulls of mosasaurs. Upper figure, Clidastes, from the side; middle figure, Platecarpus, from below; lower figure, Tylosaurus, from above: an, angular; bs, basisphenoid; c, coronoid; ep, epipterygoid; fr, frontal; j, jugal; l, lacrimal; m, maxilla; na, nasal; oc, occipital condyle; pa, parietal, palatine; pm, premaxilla; pf, prefrontal; pt, pterygoid; po, postorbital; q, quadrate; sp, splenial; sq, squamosal; tr, transverse; v, vomer.
As in most other aquatic reptiles, the neck was short and strong, the vertebrae being less in number than in most other lizards. The trunk was long and slender, more especially so in the surface-swimming kinds, with from twenty-two to thirty-four vertebrae. The tail was long, no longer than the tail of some land lizards, but more powerful, and broader and flatter. It was expanded or dilated more or less toward the free end, that is, with the beginning of a terminal caudal fin, such as the more specialized ichthyosaurs and crocodiles possessed. The vertebrae were procoelous, that is, concave in front and convex behind, like those of most modern lizards and all modern snakes and crocodiles, but quite unlike the biconcave vertebrae of all other aquatic reptiles. This kind of articulation of the backbones gave greater firmness and strength to the spinal column, but decreased the flexibility, and its possession by these animals was doubtless due to their descent from land lizards which had already acquired it. The loss of flexibility, however, was partly compensated by the loss of the additional articulating surfaces of the tail.
