762 MAMMALIA. 



ought rather to be regarded as a elass by itself, is composed of numerous 

 families, of diverse forms and very opposite habits. The Opossums 

 (DidelpJiys) of the American continent live in trees, and devour birds, 

 insects, or even fruits : in these, the thumb of the hind foot is opposable 

 to the other fingers, and adapted for grasping the boughs, whence they 

 are called Pedimanes; their tail is likewise prehensile. Others are 

 terrestrial in their habits, wanting the prehensile thumb. 



Fig. 384. 



Skeleton of the Kangaroo Rat. 



(2190.) The Kangaroo Eat, or Potoroo (Hypsiprymnus), of whose 

 skeleton we have given a drawing (fig. 384), is remarkable for the dis- 

 proportionate size of its hind legs : these, moreover, have no thumb, 

 and the two innermost toes are joined together as far as the nails ; so 

 that there appear to be but three toes, the inner one being furnished 

 with two claws. Such legs are well adapted to make strong and 

 vigorous leaps over a level plain ; and in the Kangaroos (Macropus) the 

 extraordinary development of the posterior extremities is even yet more 

 wonderful. In other respects, the skeletons of the Marsupialia conform 

 to the general description already given. 



(2191.) All other MammiferousVertebrata produce their young alive, 

 and not until they have attained a considerably advanced state of deve- 

 lopment during their mtra-uterine existence. The connexion between 

 the maternal and foetal systems in these orders is maintained during 

 the latter periods of gestation by the development of a peculiar viscus, 

 called the placenta ; nevertheless, after birth, the young animals are 

 still dependent upon the mother for support, and live upon the milk 

 supplied by her mammary organs. 



