THE SKULL 91 



The roofing bones of the skull are well-developed and in the 

 Lacertilia may become closely united with overlying dermal bones, 

 while the trabecular region (ali- and orlitosplicnoids) becomes of 

 secondary importance in the adult, its place being partly taken by 

 processes growing downwards from the frontal and parietal : 

 this is especially the case in Snakes. 



The parietals are paired in the Chelonia and in Hatteria ; in 

 all other Keptiles they become fused together, as do also the 

 frontals in many Lizards and Crocodiles. A parietal foramen 1 is 

 present in many Lizards. 



The topographical relations of the several bones to one another 

 are shown in Figs. 71 to 74. It will be seen in them that the 

 ground-plan of the Urodele skull is here fundamentally retained. 

 In addition, however, to a postorbital, an imperfect circumorbital 

 ring of bones is present in Lizards. In many Lizards, moreover, 



"Coee 



FIG. 74. SKULL OF YOUNG WATER- TORTOISE (Emys euroficea}. Side view. 



Osp, supracccipital, which gives rise to a crest ; Pf, prefrontal, which forms a 

 great part of the anterior boundary of the orbit ; /, point of entrance of the 

 olfactory nerve into the nasal capsule ; Na, external nostril ; Si, interorbital 

 septum ; UK, horny sheaths of jaws ; lug, jugal ; Qjg, quadratojugal ("para- 

 quadrate," Gaupp) ; Mt, tympanic membrane ; BP, cartilaginous interval 

 between basioccipital and basisphenoid ; Md, mandible. Other letters as in 

 Figs. 72 and 73. 



a rod-like bone, the cpipterygoicl (also represented in Crocodiles), 

 connects the parietal with the pterygoid, and a transverse lone 

 extending from the maxilla to the pterygoid is typically present 

 in Reptiles, but is wanting in the Chelonia and Typhlopidas. 



The auditory capsules are ossified from three centres, the 

 prootic usually remaining free, and the epiotic uniting with the 

 supraoccipital and the opisthotic with the exoccipital. A fenestra 

 rotunda is present in its walls in addition to a fenestra ovalis, into 

 which latter the stapedial plate of the columella is inserted (see 

 p. 84), and the tympanic cavity in most Reptiles communicates 

 with the pharynx by means of an Eustachian tube. 



1 In certain Chameleons its representative is in the frontal. 



