2l6 



THE SKULL. 



[chap. 



some ; in others it forms a short external auditory meatus, 

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

 the cranium, 



13. The periotic sends backwards a distinct mastoid, 

 which appears as a narrow strip of bone of considerable 

 vertical extent, between the squamosal 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. 



16. The optic foramen is confluent with the sphenoidal 

 fissure. 



17. The mandible has {Tarsipes excepted) an inverted 

 border to the angle. 



X 



Fig. 68. — Upper surface of hyoid of 

 Wombat {Phascolomys latifrons). bh 

 bashyal ; ch ceratohyal ; s/i stylohyal; 

 th thyrohyal. 



Fig. 69. — Hyoid of Kangaroo {Macro- 

 pus), eh epihyal ; bh basihyal ; th 

 thyrohyal. 



18. The hyoid has a small, more or less lozenge-shaped 

 basihyal, broad ceratohyals, the remainder of the anterior 

 cornu usually unossified, and stout, somewhat compressed 

 thyrohyals. 



Order Monotremata. — Both the animals of this group 

 present very singular modifications of the cranium. 



The cerebral cavity, unlike that of the lower Mar- 

 supialia or the Reptiles, with which they have so many 



