THE VERTEBRATES: MAMMALS 245 



America. In the following paragraphs some of the more 

 familiar mammals representing each of the eight orders 

 represented in North America are referred to. 



The opossums (Marsupialia).--The opossum (Didelphys 

 virginiana) is the only North American representative of 

 the order Marsupialia, the other members of which are 

 limited exclusively to Australia and certain neighboring 

 islands. The kangaroos are the best known of the foreign 

 marsupials. After birth the young are transferred to an 

 external pouch, the marsupium, on the ventral surface of 

 the mother, in which they are carried about and fed. The 

 opossum lives in trees, is about the size of a common cat, 

 and has a dirty-yellowish woolly fur. Its tail is long and scaly, 

 like a rat's. Its food consists chiefly of insects, although 

 small reptiles, birds, and bird's eggs are eaten. When ready 

 to bear young the opossum makes a nest of dried grass in the 

 hollow of a tree, and produces about thirteen very small (half 

 an inch long) helpless creatures. These are then placed by 

 the mother in her pouch. Here they remain until two 

 months or more after birth. Probably all the North Ameri- 

 can opossums found from New York to California and espe- 

 cially common in the Southern States belong to a single 

 species, but there is much variety among the individu- 

 als. 



The rodents or gnawers (Glires).--The rabbits, porcu- 

 pines, gophers, chipmunks, beavers, squirrels, and rats and 

 mice compose the largest order among the mammals. They 

 are called the rodents or gnawers (Glires) because of their 

 well-known gnawing powers and proclivities. The special 

 arrangement and character of the teeth are characteristic of 

 this order. There are no canines, a toothless space being 

 left between the incisors and molars on each side. There are 

 only two incisor teeth in each jaw (rarely four in the upper 

 jaw), and these teeth grow continuously and are kept sharp 

 and of uniform length by the gnawing on hard substances 



