NIPPLES OF PLACENTALIA 97 



amongst the Marsupialia in the genera Peramys 

 and Marmosa, but unpaired nipples and the 

 four-rowed arrangement are never met with. 

 Very frequently the nipples are arranged in 

 single rows all along the abdomen, from the 

 axilla to the inguinal region — as, for example, 

 in the pig and the dog. In many cases the 

 middle region of the abdomen is free from 

 nipples, as in Mus muscuhcs, where there are 

 three pairs of nipples on the breast, and two 

 pairs in the inguinal region. Often only the 

 caudal parts of the nipple rows are developed, 

 most conspicuously in the ruminants and the 

 Perissodactyls, where the nipples of the right 

 and left sides are situated together on the so- 

 called udders. On the other hand, the nipples 

 are often found only on the breast, as, for in- 

 stance, in man, the anthropoid and catarrhine 

 monkeys and also in elephants, many of 

 the Edentata, bats, etc. The nipples of many 

 South American rodents are very singularly 

 placed, those, for instance, of the Coypu 

 (Myopotamus), Viscacha (Lagostomus), and 

 Chinchilla, being situated a little above a 

 line drawn between the origins of the fore 

 and hind limbs (Fig. 36). In the hippo- 

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