MAMMALS. 369 



are nourished by milk secreted by the mother, as in the 

 case with all other mammals. 



The Mammalia are divisible into two classes: Mono- 

 tremata and Eutheria. 



CLASS I. MONOTREMATA. 



This class contains three or four species of animals 

 which are found only in Australia and its immediate 

 neighborhood. They present resemblances to the birds, 

 or, better, to the reptiles, in the following points in all of 



FIG. 159. Duckbill (Ornithorhynchus paradoxus}. From Liitken. 



which they differ from the other mammals: They lay 

 eggs; they have well-developed coracoid bones; the 

 bones of the skull are fused, as in birds, and reproductive 

 and excretory organs empty into the posterior portion 

 (cloaca) of the intestine, and thence pass by a common 

 opening to the exterior. 



The monotremes include the duckbill (fig. 159) and the 

 spiny ant-eaters. The duckbill is an aquatic animal, and 

 receives its common name from the fact that it has a 



