68 MAMMARY APPARATUS OF THE MAMMALIA 



in the reverse case, a reduction of the number 

 of nipples occurs — as is characteristic of many 

 of the Didelphyidag — it always happens through 

 the disappearance of the more cranial nipples, 

 and the outer as well as the inner rows may 

 be affected. In the latter case, if the primordia 

 8 and y are suppressed, there would result only 

 two simple rows of nipples, enclosing between 

 them an unpaired central nipple. In the other 

 eventuality, when the primordia d and e are sup- 

 pressed, we have actually four rows of nipples, 

 which, however, are so arranged as to appear 

 like two. 



If now we pass from these theoretical pos- 

 sibilities to the examination of the specimens 

 themselves, the significance of the above con- 

 siderations at once becomes clear. For we 

 learn that in all genera of the Didelphyidas, a 

 four -rowed arrangement of nipples exists, 

 although hitherto it has not been recognized. 



Fig. 24a and Fig. 24b illustrate the nipple 

 arrangement in a Marmosa ■murina, belonging 

 to the British Museum. According to the 

 usual conception, the nipple formula hefe is 

 5-1-5, in spite of the peculiar position of the 

 cranial nipples not fully everted out of their 



