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 

 present as bones; but the j mx j mx 



basi-occipital (Tortoises) and 

 the supra-occipital (Croco- 

 diles and Snakes) may be 

 excluded from the boundary 

 of the fora men magnum. In 

 the periotic capsule there is 

 a fenestra rotunda, as well 

 as the fenestra, ovalis with 

 the coluniella. 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, on 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 

 basisp/ienoid 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 Chelonia 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. 631. Skull of Monitor (after Gegenbaur). a, 

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

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

 oc^ipital; P, parietal; Fr, frontal; Pf, post- 

 frontal ; Prf, preefrontal ; L, lacrymal ; 2V, nasal; 

 Sq, squamosal ; Q, quadrate ; Q,i, quadratojugal ; 

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

 Co, columfllla; Bs, basi-sphenoid ; ft, pterygoid; 

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



