ISO STRUCTURAL AND SYSTEMATIC ZOOLOGY 



SUBCLASS II. Theria 



In these mammals the mammary glands are typically 

 developed and there is no cloaca. 

 There are two sections : 



SECTION A. Metatheria 



In this group are the marsupials, opossums, bandi- 

 coots, wombats, and kangaroos. 



Marsupialia are distinguished by the fact that the 

 young, always born premature, are transferred by the 

 mother to a pouch on the abdomen, where they are 

 attached to the nipples, and the milk is forced into 



FIG. 167. Virginia opossum (Didelphys virginianci). 



their mouths by special muscles. 48 They have " mar- 

 supial bones" projecting from the pelvis, which may 

 serve to support the pouch ; though as the monotremes 

 have the same bones, but no pouch, they doubtless 

 have some other function. These bones are peculiar 

 to animals having no placenta, namely, to monotremes 

 and marsupials. The brains of marsupials resemble 

 those of the monotremes, except that the cerebrum of 



