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. 



sh i 



FIG. 72. Upper surface of hyoid 01 

 Wombat (Phascolomys latifrons). bh 

 basihyal ; ch ceratohyal ; sh stylohyal ; 

 tk thyrohyal. 



FIG. 73. Hyoid of Kangaroo (J\Iac>-t>- 

 piis). 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 paroccipital 

 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. 



