54 2 Bulletin American Museum of Natural History. [Vol. XIX, 



August, September, and October. The April and May adults, 

 and the September and October specimens are much darker, 

 or more blackish, and less faded than the July and August 

 specimens, which are in moult and in thin pelage in com- 

 parison with the thick, soft, and longer coat of the April-May 

 and September-October examples. 



The young are born, judging from the present material, 

 mainly during July and the first half of August; April and 

 May females not yet having begun to nurse young, while in 

 September and October the nipples have become so shrunken 

 and the mammae so heavily enclosed in soft fur that it is diffi- 

 cult to distinguish males from females by an examination of 

 the skins. During the nursing period, and until the comple- 

 tion of the moult, a bare space surrounds the nipples, indi- 

 cating, usually at a glance, the females that have recently 

 nourished young. 



Mr. Anderson states: "Near Telegraph, white-footed mice 

 were trapped in every kind of place to be found, wet or dry, 

 talus or level terrace." At the camp at timber-line on the 

 Shesley River they "were not common," and only three were 

 obtained on Level Mountain ; but they were very abundant at 

 the camp on Raspberry Creek. 



EXTERNAL MEASUREMENTS OF 40 BREEDING FEMALES OF 



Peromyscus arcticus. 



