120 ELEMENTS OF BIOLOGY 



Sub-class L prototheria (Fig. 72). Egg-laying mammals. No 

 nipples for feeding the young. Two Australian forms, the Duckbill 

 and the Echidna. 



Sub-class IL metatheria (Fig. 73). The Marsupials. The young 

 are born in a very immature state and complete their development 

 in a pouch or marsupium on the ventral body wall of the female. 



Ay£S 



FIG. 71 BIRD 

 Fig. 71, — A common bird, representative of the class Aves. Compare the beak 

 and claws with those of a reptile (Fig. 70). For the internal structure of a bird, 

 see Fig. 212. 



The Marsupials include such forms as the Kangaroo and the 

 Opossum. The Opossum is the only North American representa- 

 tive of this group. 



Sub-class in. eutheria. All other Mammals except those de- 

 scribed in the above two sub-classes. There are nine modern orders 

 of Eutheria: 



