00 



COMPARATIVE ANATOMY 



of ossification, which gives the skull a very firm and solid appear- 

 ance ; only amongst Lizards (Fig. 71), and especially in Hatteria 

 is the cartilage retained to any considerable extent, and owing to the 

 conformation of the bones in the posterior region, the skull in these 

 forms presents a number of distinct spaces or fossae in the dry state. 



In Snakes and Amphisbsenians the cranial cavity extends 

 forwards between the orbits as far as the ethmoidal region, while 

 in the Lacertilia, Chelonia, and Crocodilia — in which a fibro-carti- 

 laginous interorbital septum perforated by the olfactory nerve is 

 present — its anterior boundary is much further back. 



The parasphenoid, which plays so important a part as an 

 investing bone of the roof of the mouth in Fishes and Amphibians, 



J^thFm.^ 



Pmx 



■Foi/ 



Ol COCD 



(fa 



^rt^ 



Fig. 72. — Skull of Snake {Tropidonotus natrix), dorsal view. 

 Tig. 73. — ,, ,, ,, ,, ventral view. 



Cocc, occipital condyle ; Osp, .supraoccipital ; 01, exoccipital ; Foe, fenestra ovalis ; 

 Pe, periotic ; P, parietal ; F, frontal ; i''^, postfrontal ; Pf, prefrontal ; 

 Mh, ethmoid ; N, nasal ; Pmx, premajcilla; M, maxilla ; Bp, basioccipital ; 

 Bs, basisphenoid ; Ch, posterior nostrils ; Vo, vomer ; PI, palatine ; Pt, 

 pterygoid; Ts, transverse bone; Qii, quadrate; Squ, squamosal; Art, 

 articular ; Ag, angular ; SA, supra-angular ; Dt, dentary ; II, optic foramen. 



begins to disappear ; amongst Eeptiles it only attains any im- 

 portant development in Snakes, where the anterior part remains 

 and forms the base of the interorbital region. Its place is taken 

 by two cartilage bones, the basioccipital and basisphenoid, situated 

 along the basis cranii. In contradistinction to the Amphibia, only 

 a single condyle connects the skull with the vertebral column: 

 this, on close examination, is seen to be formed of three parts, 

 derived from the basioccipital and exoccipitals respectively. 



