xi ii. ] EDENTA TAMARSUPIAL1A. 235 



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 

 ankylosed with the maxillae. The pterygoids are much 

 smaller, but sometimes are bullate. 



The upper margin of the mandible is produced anteriorly 

 into a spout-like process. The condyle is scarcely above 

 the level of the molar teeth, and is wide from side to side. 

 The angular projection is smaller and thicker. 



The hyoid (in an old specimen) has a strongly ossified 

 anterior arch, consisting of two bones of nearly equal length, 

 the proximal one with a stout-rounded process projecting 

 backwards and outwards from near its upper end. The 

 second is bent at a right angle near its lower end, and may 

 result from the ossification of two elements. The basi- and 

 thyro-hyals are ankylosed to form a wide U-shaped bone. 



Order MARSUPIALIA.- -The skull of the large carnivorous 

 Marsupial, the Thylacine (Thylacinus cynocephalus\ resembles 

 so closely that of a Dog in its general aspect, that it will be 

 well to commence an account of the peculiarities of the 

 crania of Marsupials generally by comparing these two 

 skulls. 



It will be seen by the section (Fig. 70) that the brain cavity 

 of the Thylacine is very much smaller than that of the Dog 

 (Fig. 46, p. 117) in relation to the size of the rest of the 

 cranium, or to that of the whole animal, a sign of great in- 

 feriority of organisation. This diminution affects chiefly the 

 cerebral fossa ; the cerebellar fossa is nearly equal in size, 

 but it is placed more directly behind the cerebral, and is 



