MAMMALIA. 209 
7 
(Fig. 323, 8). They live upon worms and vegetable mat- 
ter. Their nests are long burrows in the banks of streams, 
Fic. 323.—4, head of Ornithorhynchus, showing serrated bill; 2, hind-foot 
with spur, 2, 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.—D1IpeELpuHia. 
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 they 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 tothe front Fie, 324.—Opos- 
of the pelvis. sum at birth, 
Opossum (Didelphide).—In this fam- 
ily is the common opossum (Fig. 331), the only marsupial 
of the United States. It is about twenty inches in length, 
