210 THE SKULL. [chap. 



zygomatic process of the squamosal. The nasals are short 

 and wide. The anterior nares are nearly vertical, or rather 

 inclining downwards. The premaxillae are exceedingly 

 rudimentary, only the palatal portion being present, without 

 any ascending process j they unite with each other across 

 the middle line, but not with the maxillae, hence they are 

 generally lost in macerated skulls. The palate is narrow, 

 especially posteriorly, and not produced behind the molar 

 teeth. The pterygoids form large plates with prominent 

 rounded borders, compressed in some, and inflated in 

 other species. The glenoid fossa is narrow from side to 

 side. The tympanic, squamosal, and periotic are ankylosed 

 together. The former forms a considerable bulla, but no 

 tubular meatus. There are large supratympanic air sinuses 

 and a well-marked ossified tympanohyal. 



The mandible has a comparatively high horizontal por- 

 tion, rounded in front with a very small median triangular 

 process at the upper border. The coronoid process is high 

 and slender. The condyle is small ; its articular surface is 

 convex from side to side, short and nearly straight from before 

 backwards. The angle forms a broad compressed posterior 

 projection, with a slightly incurved lower border. 



The stylohyals are large, compressed, and curved, with a 

 prominent posterior process near their upper end. The 

 basihyal is small, and ankylosed with the thyrohyals, so that 

 they form together a V-shaped bone. 



The skull of the Two-toed Sloth (Cholcepus didactylus), 

 though generally similar to the last, presents in some points 

 marked deviations from it. Even in aged specimens, in 

 which almost all the sutures are obliterated, the tympanic 

 is a mere ring, incomplete at the upper margin, and but 

 slightly connected with the other bones around. The 

 premaxillae are more developed, and become ultimately 



