CHAPTER XXI 

 SUBPHYLUM VERTEBRATA: CLASS VII. MAMMALIA 



The mammals are popularly known as " animals." The name 

 of the class is derived from the fact that most mammals possess 

 mammary glands which secrete milk for the nourishment of their 

 young. Mammals also possess a covering of hair at some time 

 in their existence and are distinguished by this characteristic as 

 certainly as birds are by their feathers. With few exceptions 

 adult mammals are provided with at least a small number of 

 hairs. 



The seventy-five hundred or more species of living mammals, 

 and the three thousand or more species of fossil mammals may 

 be grouped into two subclasses, (i) Prototheria, or egg-laying 

 mammals, and (2) Eutheria, or viviparous mammals. 



The three living genera of the Prototheria are included in 

 one order which is confined to Australia, Tasmania, and New 

 Guinea. They are the spiny ant-eater and duckbills (Fig. 513). 



The Eutheria may be grouped into two divisions : — 



I. Didelphia, or marsupials, such as the opossum and kanga- 

 roo, with a pouch in which the young are carried after birth, and 

 without a typical placenta (see p. 641). 



II. Monodelphia, or placentals, with a typical placenta 

 before birth, and more highly developed young. 



The Monodelphia may be subdivided into four sections: — 



(A) Unguiculata, or clawed mammals, such as the moles, 

 bats, dogs, cats, seals, squirrels, mice, ant-eaters, and sloths. 



(B) Primates, with fingers usually terminating in " nails," 

 such as the lemurs, monkeys, apes, and man. 



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