THE SKELETON IN MAMMALIA. THE SKULL. 457 



Thylacinus and the dog. The skull of Notoryctes is strongly sug- 

 gestive of that of an Insectivore, and that of other Marsupials 

 such as the wombat, recalls equally the characteristic features 

 of a Rodent's skull. But, however much they may differ from 

 one another, the skulls of all Marsupials agree in the following 

 respects. (1) The brain cavity, and especially the cerebral 

 fossa, has a very small comparative size. (2) The nasals are 

 always large, and the mesethmoid is extensively ossified, and 

 terminated by a prominent vertical edge. (3) Processes from 

 the jugal and frontal in living forms never meet and enclose 

 the orbit, but the zygomatic arch is always complete. (4) The 

 jugal always extends back to form part of the glenoid fossa. 

 (5) The lachrymal canal opens either external to or upon the 

 margin of the orbit, and the nasal processes of the premaxillae 

 never quite reach the f rentals. (6) The posterior part of the 

 palate is commonly pierced by large oval vacuities. (7) The 

 tympanic is small and never fused to the bones of the 

 cranium. (8) The carotid canal perforates the basisphenoid 

 and not the tympanic bulla. (9) The optic foramen and 

 sphenoidal fissure are confluent. (10) In every case except 

 Tarsipes the angle of the mandible is more or less inflected. 



The skull of the extinct Thylacoleo differs from that of all 

 other Marsupials in the fact that the postorbital bar is com- 

 plete. The hyoid is constructed on much the same plan in all 

 Marsupials. It consists of a small basi-hyal, a pair of broad 

 cerato-hyals, and a pair of strong thyro-hyals. The epi-hyals 

 and stylo-hyals are generally unossified. 



EDENTATA. In Sloths (Bradypodidae) the sutures become 

 early obliterated, the cranial portion of the skull is rather 

 high, and the facial portion very short. The lachrymal is very 

 small, and its canal opens outside the orbit. The zygomatic 

 arch is incomplete, and the jugal (fig. 91, 5) is curiously forked, 

 but in a manner differing in the two genera. The premaxillae 

 are very small, in Bradypus quite vestigial. The mandible is 

 well developed, the angle being specially marked in Bradypus. 



