The adaptation of the mosasaurs to an aquatic life was very complete, though perhaps not so complete as was that of the ichthyosaurs. The skull is flattened, narrow, and more or less elongate, but large in proportion to the remainder of the skeleton—nearly one-sixth of the entire length; that relative size doubtless is indicative of very predaceous and pugnacious habits. The teeth in the typical forms are numerous, strong, and sharp, conical in shape, and recurved. Not only are there numerous teeth in both the upper and lower jaws, but there are also two rows of strong teeth implanted in the back part of the palate, upon bones called pterygoids, the use of which will be understood later. The teeth were inserted on large, tumid, bony bases, rather loosely attached in shallow pits or alveoli, unlike the teeth of all modern lizards. Such a mode of attachment of the teeth doubtless had some relation to the habits of the animals concerning which we are not quite clear. They were easily dislodged, and, in consequence, of very unequal size, some full grown, some small, and others just appearing above the surface of the gums in the living animals. The frequent loss of teeth and their constant and easy replacement by new ones is a peculiarity of predaceous reptiles, thereby insuring their best functional use. 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. [Illustration: FIG. 71.—_Clidastes_, inner side of right mandible: _ang_, angular; _art_, articular; _cor_, coronoid; _pa_, prearticular; _sur_, surangular.]