THE SKULLS OF EEPTILIA AND AVES. 



223 



shelters the summits of the vertical semicircular canals. l>ut 

 the study of development has shown that this part of the supra- 

 occipital is, in many, if not all, Reptiles and Birds, developed from 



Fig. 89. 

 A 



BO 



Fig. 89. — External view of the auditory region of the skull in (A) a Crocodile (C. biporcatus), 

 and (B) a Turtle {Chelone midas). The walls of the tympanic cavity have been cut 

 away in each case so far as is necessary to show the auditory fenestra ; and, in the 

 Turtle's skull, the semicircular canals are also partially displayed. In the Crocodile's 

 skull (A) F.O. is the fenestra ovalis, separated by the cochlear process of the opis- 

 thotic, c, from the fenestra rotunda ; Chi is the hook formed by the curved process 

 (6) of the opisthotic, which supports the cochlea externally. The lower end of the 



