lO THE OSTEOLOGY OF THE REPTILES 



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 ptero- 

 saurs. 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 a), 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 immedi- 

 ately 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 character- 

 istic 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 



