MARSUPIALIA. 99 



The family of Orycteropodidce contains one genus only, the 

 Aard-varks, which have ^^ teeth, of a highly peculiar and com- 

 plex structure, each tooth consisting of a very large number of 

 separate parallel dental systems, all closely packed together. In a 

 transverse section they present an appearance not unlike that of a 

 piece of cane. These teeth are preceded by a set of minute milk- 

 teeth, mere remnants of a former functional set, which show indi- 

 cations of a division into different groups, such as premolars 

 and molars. The zygoma is complete, and there are well-deve- 

 loped clavicles. 



The Common Aard-vark (Orycteropus afer), an animal of about 

 the size of a pig, is represented by a skeleton in Case 23, Div. E. 



Order XL MARSUPIALIA, or POUCHED ANIMALS. 



The principal characteristic by which the skeletons of Marsu- 

 pials differ from those of all the previous Orders is the presence of 

 a pair of long slender bones, attached to the anterior edge of the 

 pelvis. These are known as the " marsupial bones," owing to their 

 close neighbourhood to the external marsupial pouch *. In their 

 skulls the Marsupials differ from other Mammals by having the 

 angle of the lower jaw much bent inwards, and forming a well- 

 marked internal process. Teeth are always present, and separable 

 into different classes, but, with the exception of the last premolar, 

 not preceded by milk-teeth. The incisors are generally unequal 

 in number in the two jaws, and range from to ^. Dorso-lumbar 

 vertebrae invariably 19. 



The Marsupials are divided primarily into two great groups : 

 in the first, as in the Rodents and Ungulates, the incisors are few 

 in number, but large and powerful, and the canines, at least in 

 the lower jaw, are either entirely absent or small and rudimentary ; 

 while the second possess, like the placental Carnivora, small and 

 numerous incisors and large and sharp canines. The former are 

 called "Diprotodont"f and the latter " Polyprotodont "J. 



To the former group belong the Kangaroos, Phalangers, and 



* They are absent in one genus only, Thylacinus. 



t " With two front teeth." J "With many front teeth. , 



H2 



