No thalattosaurs were large animals. If they had the same proportions between the lengths of head, body, and tail as the mosasaurs, none exceeded seven feet in length, and they may have been even shorter, though probably not much. The figure of the skull, as restored by Professor Merriam, shows many striking aquatic adaptations, in the elongated, pointed muzzle, in the large external nostrils, situated far back toward the eyes, and in the well-ossified ring of bones surrounding the eyeball. There is a parietal opening in the roof of the skull, as in the modern lizards and tuatera; but it is not known for certainty whether there were two openings on each side in the roof of the skull, as in the modern tuatera. While this character may seem trivial, it is really one of the most important in the reptilian anatomy in determining the relationship and classification of reptiles. The teeth are conical and pointed in the front end of the upper and lower jaws, but farther back they are rounded, rugose, and obtuse, and could have been used only for crushing hard objects, like mollusks, crustaceans, etc. (Fig. 82). And not only was there a row of such teeth on each jaw (only partly seen in the figure), but similar teeth covered a large part of the palate. And the lower jaws, it is seen, are rather massive. The vertebrae were, of course, of the more primitive kind, that is, with the ends concave, both in front and behind. It would have been strange indeed were they of any other kind, since reptiles with ball-and-socket joints to the vertebrae, that is, concave on one end and convex on the other, as in nearly all living reptiles, did not come into existence till long after the thalattosaurs had disappeared from geological history; and it is also a curious fact that such vertebrae appear to have originated only among animals crawling on land, so that they would not have been a character acquired by the thalattosaurs after descending into the water. It will be seen from the figure of a dorsal vertebra that the rib was attached by a single articular surface, almost exclusively to the body of the vertebra, quite like those of all lizards, snakes, and mosasaurs, and unlike those of other reptiles. This too may seem to be a trivial character to prove relationships with the lizards, but it is a curious fact that no two animals having different kinds of ribs are closely related to each other. Possibly, however, this looser mode of attachment of the ribs in the thalattosaurs was one of their peculiar adaptations to a water life, and may not have been derived from their land ancestors. [Illustration: FIG. 83.—Dorsal vertebra of _Thalattosaurus_. (After Merriam.)] [Illustration: FIG. 84.—_Thalattosaurus_: bones of front extremity: _s_, scapula; _c_, coracoid; _h_, humerus; _r_, radius; _u_, ulna. (After Merriam.)]