288 COMPARATIVE ZOOLOGY OF CHORDATES 



anterior border which is also the hind border of the orbit ; 

 a lower border, reaching from the maxilla to the quadrate ; 

 and a posterior border which is also the upper border of the 

 post-temporal fossa. 



The most primitive reptiles or Cotylosaurs of which 

 Seymouria is an example, are characterised by skulls of this 

 type, in which the temporal cavity is completely roofed over ; 

 a condition inherited from the Stegocephalian ancestors. 



In the Chelonia probably the skull was primitively of this 

 kind also, and Chelone is a good example of a skull with a 

 temporal cavity completely roofed over, opening behind by a 

 post-temporal fossa.* In other forms of tortoises and turtles, 

 however, the roof over the temporal cavity becomes reduced 

 by a process known as emargination. The skull-roof becomes 

 as it were eaten away from the edge, and this reduction may 

 affect the hind border or the lower border of the roof of the 

 temporal region, or both. When reduction by emargination 

 has taken place, the prootic and opisthotic bones of the auditory 

 capsule become visible from the dorsal side of the skull. It is 

 important to notice that in emargination there is no perforation 

 of the skull-roof. 



It is common to find the Cotylosaurs and the Chelonia 

 grouped together as Anapsida, since they have skulls completely 

 roofed-over or sometimes emarginated, but never perforated 

 as regards the roof by apertures other than the orbits and 

 nostrils. These forms usually have three condyles. 



The remaining vertebrates are characterised by the fact 

 that the roof of the skull in the temporal region has been 

 perforated, with the result that windows are formed, completely 

 surrounded by bone, and opening into the temporal cavity. 

 A window of this kind is called a temporal fossa or vacuity, and 

 it enables the muscles of mastication to become enlarged. 

 Through the window the auditory capsule is visible. It must 

 be clearly understood that a temporal fossa is only a perforation 



* It should be mentioned that some authorities prefer to regard the 

 complete roofing of Chelone as secondarily developed. This is immaterial 

 for the present purpose, which aims only at pointing out the typical 

 relations of the temporal region of the skull. 



