MAMMALIA. 



299 



(Fig- 323, &). They live upon worms and vegetable mat- 

 ter. Their nests are long burrows in the banks of streams, 



FIG. 323. A, head of Ornithorhynchus, showing serrated bill ; B, hind-foot 

 with spur, a, found on the males only ; C, webbed fore-foot. 



having an opening under water. At the farther end, twenty 

 or thirty feet from the water, leaves and grass are placed, 

 two eggs at a birth deposited, and the young reared 



Sub-Class II. DIDELPHIA. 



Order I. Pouched Animals (Marsupialia). Gen- 

 eral Characteristics. In these animals the young are born 

 in an immature state, in the great kangaroo being not over 

 an inch in length, and immediately placed 

 in a pouch or marsupium, where the.y re- 

 main attached to the teats at the bottom 

 of the pouch, the milk being forced down 

 the throat by the muscular action of the 

 mother. The young are prevented from 

 suffocating by a peculiar modification of 

 the breathing-organs. The pouch is sup- 

 ported by two long, slender bones project- 

 ing forward from and attached to the front 

 of the pelvis. 



Opossum (Didelphidce). In this fam- 

 ily is the common opossum (Fig. 331), the only marsupial 

 of the United States. It is about twenty inches in length, 



FIG. 324. Opos- 

 sum at birth. 



