240 



THE SKULL. 



[CHAP. 



bers of the group it can scarcely be said to form a distinct 

 bulla, but in some of the Hypsiprymni (Rat Kangaroos) it is 

 immensely expanded. In the Koala (Phascolarctos)^ the 

 alisphenoid bulla is very large, elongated vertically and 

 compressed, having a very similar appearance in fact to the 

 tympanic bulla of the Pig. 



12. The tympanic is small, simple, and annular in some ; 

 in others it forms a short external auditory meatus, but 

 it is never ankylosed to any of the other bones of the 

 cranium. 



FIG. 72. Upper surface of hyoicl oi 

 Wombat (fhascolomys latifrons). bh 

 basihyal ; fh ceratohyal ; sh stylohyal ; 

 tk thyrohyal. 



K;. 73. Hyoid of Kangaroo (Alacrn- 

 J>us). bh basihyal ; ch ceratohyal ; th 

 thyrohyal. 



13. The periotic sends backwards a distinct mastoid, which 

 appears as a narrow strip of bone of considerable vertical 

 extent, between the squarnosal and exoccipital, on the side 

 of the occipital region of the skull. 



14. There are almost always conspicuous paroccipita-1 

 processes. 



15. The internal carotid artery perforates the basi- 

 sphenoid. 



1 6. The optic foramen is confluent with the sphenoidal 

 fissure. 



17. The mandible has (Tarsipes excepted) an inverted 

 border to the angle. 



