THE SKULL. 



197 



vertebrae, of which there are usually two, have very large transverse 

 processes and ribs. 



The skull (fig. 631) articulates with the atlas by means of an un- 

 paired, often trifid condyle of the occipital bone, and presents a complete 

 ossification of nearly all its parts, the primordial cranium being almost 

 completely replaced. In the occipital region all four elements are 



Jmx 



Jmx 



Tr 



present as bones; but the 



basi-occipital (Tortoises) and 



the supra-occipital (Croco- 

 diles and Snakes) may be 



excluded from the boundary 



of th^ for amen magnum. In 



the periotic capsule there is 



a fenestra rotunda, as well 



as the fenestra ovalis with 



the columella. The opisthotic, 



which usually fuses with the 



exoccipital, takes part in 



bounding the fenestra ovalis 



(in the Tortoises the exocci- 

 pital and the opisthotic are 



separate). 



The prootic } \^)u the other 



hand, is separate in all Rep- 

 tiles; at its anterior there is 



in front of the lateral parts of 



the occipital region a separate 



prootic, at the front margin 



of which is the foramen for 



the third branch of the tri- 



geminus. The epiotic is 



fused with the supra-occipi- 

 tal. The anterior expansion 

 of the cranial capsule, and the development of the sphenoidal region, 

 present considerable differences. At the base of the skull there is a 

 basisphenoid in place of the parasphenoid. The alisphenoids and the 

 orbitosphenoids are as a rule wanting, and are often replaced by pro- 

 cesses of the parietals (Chelonia) or of the fronto-parietals (Ophidia). 

 In the Chdonia and Lizards there is a large membranous interorbital 

 septum, which may also contain ossifications. The bones of the roof 

 of the cranium are always very large sometimes paired, sometimes 



FIG 



- Skull of Monitor (after Gegenbaur). a, 



from above, b, from below. C, occipital condyle ; 

 Ocs, supra- occipital; Ocl, exoccipital; Ocb, basi- 

 oc-npital; P, parietal; Fr, frontal; Pf, post- 

 frontal ; Prf, praefrontal ; L, lacrymal ; N, nasal; 

 Sq, squamosal ; Q, quadrate ; Qi, quadratojugal ; 

 J, jugal ; MX, maxillary ; Jmx, praemaxillary ; 

 Co, columella; Bs, basi-sphenoid ; Pt, pterygoid ; 

 Pal, palatine ; Vo, vomer ; Tr, os transversum. 



