SKULL REPTILES 



147 



occipital sometimes excluded from the foramen magnum. The tri- 

 partite condyle is formed by basi- and exoccipitals. In recent species 

 the supraoccipital is prolonged backwards in a strong occipital spine, 

 the parietals being continued along its base, these latter bones being 

 separate and almost always (Dermochelys excepted) each has a verti- 

 cal descending process which reaches the pterygoid, closing the 

 cranial cavity in front. The paired frontals occasionally extend to 

 the naris, which is otherwise bounded by premaxillae, maxillae and 

 prefrontals, nasals being absent, except in 

 Hydromediisa (fig. 147) and its alhes. A lac- 

 rimal is absent and the premaxillae are usually 

 separate. 



The orbit is bounded in front by maxilla 

 and prefrontal; above (usually) by the frontal, 

 while it is separated from the temporal fossa 

 by zygomatic and postorbital. The temporal 

 region shows two conditions. In marine turtles 

 (Cryptodira, fig. 155) postorbital and squamosal 

 extend in an arch over the origin of the muscles 

 of the lower jaw and reach the parietal, the floor 

 of the enclosed cavity being formed by parietal, 

 otics, squamosal and quadrate bones (fig. 156). 

 In other Chelonia (fig. 157) postorbital and 

 squamosal are separate from the parietal, and 

 pro- and opisthotic are visible from above, 

 while the orbit is separated in some genera 

 from the muscular pit by a bar of postorbital 

 and zygomatic; olhers have no such bar, these 

 two bones forming part of the floor of the fossa. 

 Sometimes a quadratojugal is present between 

 squamosal and zygomatic, sometimes it is 

 absent. 



As stated above, the cranial cavity is closed 

 in front by the descending plates of the parietals, 

 the alisphenoid usually not being ossified, but a 

 few species have a bone which answers to it. 

 Fibrocartilage separates the two orbits, this septum being supported 

 by a cartilage.rostrum, the common trabecula. The septum continues 

 forward into the nasal region. 



FtG. 156. — Skull of 

 Chelone with roof of 

 temporal fossa removed 

 (Butschli, '10). dp, de- 

 scending process of pari- 

 etal; eo, exoccipital; /, 

 frontal; m, maxilla; oo, 

 opisthotic; p, parietal; 

 pf, prefrontal; pi, pala- 

 tine; prn, premaxilla; po, 

 postf rental; poo. prootic: 

 pt, pterygoid; q, quad- 

 rate; so, supraoccipital; 

 sq, squamosal; z, zygo- 

 matic. 



