SKULL REPTILES 



141 



There are usually large vacuities between certain of these floor 

 bones, and the position of the choanae is very variable; usually they 

 are far forwards, the anterior ends of the vomers lying between them, 

 or the choanas of the two sides are approximate. From this primitive 

 condition all others are derived. Palatal processes from premaxilla 

 may extend medially, passing below the primitive choanas and cutting 

 off a respiratory duct from the roof of the mouth, so that the defini- 

 tive choanae are carried back to the edge of the hard palate 

 thus formed. Then palatines and pterygoids may meet in the same 

 way in the middle line, so that the 

 choanae are still further back, the 

 extreme occurring in modern Croco- 

 dilia (fig. 178) where these openings 

 are near the posterior end of the cra- 

 nium. This meeting of palatines and 

 pterygoids covers vomers and sphe- 

 noids so that they are excluded from 

 the roof of the mouth. 



The cranial cavity (fig. 149) is 'tauiL 

 bounded posteriorly by the occip- 

 italia. The otica form its postero- 

 lateral walls. Of the otica the exact 

 relations are known in a few groups. 

 Chelonia, Sphenodon, and older 

 Crocodilia have a distinct opisthotic: opisthotic; pt, pterygoid; so, supraoc- 



cipitaL 



elsewhere it appears in ontogeny, 



fusing later with the exoccipital, forming a parotic process ; it fuses 

 above with the prootic which forms the anterior wall of the otic 

 capsule. Whether an epiotic be present is questionable, the bone 

 often given that name is the tabulare. 



Farther forwards the lateral cranial wall is formed by ahsphenoids 

 (lacking in Chelonia and Ophidia) which also form part of the posterior 

 wall of the orbit and extend dorsally to frontal and parietal. In 

 Chelonia the parietal sends a broad process down to the basi- 

 sphenoid, closing the orbit from the brain cavity. Similar processes 

 from the frontal extend the medial wall farther forwards. Most 

 reptiles have a well-developed interorbital septum (nearly or quite 

 lacking in Chelonia and Ophidia), usually of membrano-cartilage, 



Fig. 149. — Internal surface of 

 posterior part of cranium of Alligator 

 (Pouchet et Beauregard, '8q). as, 

 alisphenoid; ho, basioccipital; bs, 

 basisphenoid; e, epiotic; eo, exoc- 

 cipital; fr, frontal; /, hypophysial 

 fossa; pa, parietal; po, prootic; oo. 



