138 



VERTEBRATE SKELETON 



arcade, as in snakes, results in the disappearance of all vacuities, in 

 snakes probably the result of the looseness of the squamosal and the 

 great mobiHty of the quadrate. Usually the fossae are separated 

 from the orbit by an ascending orbital process of the zygomatic 

 which meets the bones (postorbital, postfrontal, when these are 

 present) which bound the orbit behind, but when these are absent, 

 the orbital process usually extends to the frontal. In some fossil 

 reptiles (Theromorphs, Dinosaurs, Pterosaurs, figures 145, 180) there 

 may be vacuities in front of the orbit. 



The history and probable genealogy of the temporal fossae have been the 

 basis of several theories. One is that the supratemporal fossa arose by an exten- 

 sion forwards ^nd closure behind of the so-called auditory notch of many Stego- 

 cephals (fig. 76) and Seymouria of the Theromorphs, but this does not seem 

 probable. Another view of the homologies concerned is shown in figure 79. 

 The fossae and their arcades have been used in dividing ReptiHa into two groups, 

 Synapsida and Diapsida. In the first of these (many Theromorphs, Plesiosaurs, 

 Ichthyosaurs, some Chelonians and primitive Crocodilians) there is either a 

 stegocrotaphic cranium or a single fossa bounded medially by parietal and supra- 

 occipital, laterally by postfrontal (sometimes also by postorbital) and squamosal, 

 with below them and forming the ventral side of this arcade, zygomatic and 

 quadratojugal. In later Crocodilia, Rhynchocephalia and Dinosauria the 

 lower arcade is perforated by an infratemporal fossa, the lower boundary of 

 which is formed of squamosal and zygomatic and often the quadratojugal. 



All four occipitalia are developed, but in Crocodilia and Ophidia 



the supraoccipital may be excluded from the margin of the foramen 



magnum; in snakes and some turtles 

 the basioccipital is crowded out, the 

 foramen occasionally being bounded by 

 exoccipitals alone. The base of the 

 skull articulates with the atlas by a 

 single (median) occipital condyle, 

 largely or wholly (CrocodiHa, Ich- 

 thyosaurs) formed from the basioc- 

 cipital, but frequently the condyle is 

 tripartite (fig. 146) the exoccipitals con- 

 tributing to it, while in some reptiles 

 (some Theromorphs and Chelonians) 



the basioccipital part is reduced, the exoccipitals alone affording the 



articular surfaces. 



The roof of the cranial cavity is formed almost entirely of frontals 



and parietals, the former usually the larger and extending to the 



Fig. 146. — Base of cranium of 

 Chelone mydas. bo, basioccipital; 

 eo, exoccipital; o, opisthotic; pa, 

 parietal; pr, prefrontal; q, 

 quadrate; 5, supraoccipital; sq, 

 squamosal. 



