CHAPTER XVII. 

 BRANCH CHORDATA. 



CLASSIFICATION OF MAMMALIA. 

 SUBCLASS I. PROTOTHERIA. 



THERE are two subclasses of mammals, the Prototheria 

 and the Theria. The Prototheria nourish the young with 

 milk. They also show that they are mammals by their 

 hairy covering. But they reveal their relationship to birds 

 and reptiles by the fact that they lay eggs. 



FIG. 151. DUCKBILL. 



From Packard's Zoology, after Liitken. 



In this subclass there is but one order, the Monotremata. 

 The order contains but three or four species, and there are 

 two chief forms, the duckbill and spiny ant-eater, both lim- 

 ited to Australia and adjacent islands. 



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