Page:The Osteology of the Reptiles.pdf/28

10 nasals, as in the mammals, but in the chameleons it is formed by the maxillae. The paired facial horns are borne by the prefrontals or postorbitals. The frontals and parietals are sometimes developed into enormous crests in the dinosaurs, the supraoccipital in pterosaurs. Doubtless all such horns or protuberances were covered in life with a horny sheath.

The external nostrils (external nares) vary greatly in position. Primitively located near the extremity of the face (Figs. 2, 3, etc.), each was surrounded by the premaxilla, maxilla, nasal and lacrimal, and they almost always have the same relations with the first three of these bones, wherever located. Well separated by the premaxillae and nasals in the older reptiles, they are often confluent in later ones (Figs. 31, 32, 59, 68). They are surrounded by the maxillae in the chameleons (Fig. 55), by the nasals in the phytosaurs (Fig. 66); the nasals are often excluded from them, and the lacrimals have lost all relations with them since Permian times. In most aquatic reptiles they have receded toward the orbits, or rather the face has grown away from them, often for a long distance, as in the ichthyosaurs (Fig. 50), plesiosaurs (Fig. 46 ), proganosaurs, thalattosaurs (Fig. 61), and phytosaurs (Fig. 67). In the very slender-faced amphibious Crocodilia (Fig. 68) and Choristodera (Fig. 63), however, the nostrils retain their primitive position at the extremity of the face. They are located far back from the extremity in the volant pterodactyls (Figs. 71, 72) as in most birds.

The internal nares, or choanae, normally situated almost immediately below the external (Fig. 55), are carried back by a respiratory canal, formed by the undergrowth of the maxillae and palatines as a secondary palate, to a greater or less extent in the Cynodontia and Crocodilia (Fig. 69); in the former and in the early kinds of the latter, to the posterior border of the palatines; in the later crocodiles even into the pterygoids. A similar respiratory canal, probably separated from the cavity of the mouth by a membrane only, is characteristic of the Phytosauria (Figs. 66, 67). A partial secondary palate, formed by the union of the palatines or maxillae, with the opening only a little way back, occurs in some Chelonia and Anomodontia. In those reptiles in which the external nares are situated posteriorly, the internal nares are also (e.g., Figs. 61, 66). In the plesiosaurs only (Fig. 46), there may be a partial reversion of the respiratory