RODENTIA MURINAE HESPEROMYS LEUCOPUS. 461 



back ; the outside of the arm is sometimes pure white below the shoulders, sometimes with a 

 dusky tinge, sometimes colored like the back nearly to the wrist. The under surface of the tail 

 is always conspicuously whiter than the upper. 



Young specimens, again, differ considerably from the above description ; even after they have 

 attained their full size they may be of a plumbeous gray above, darker on the middle of the 

 back, sometimes with the faintest possible wash of brown on the sides. The under parts are 

 snowy white, but the plumbeous bases of the hairs show through, somewhat obscuring the 

 brilliancy of color in places. The boundaries of the colors are, however, the same as in the 

 adult, except that the outside of the fore leg down to the wrist is usually lead-colored. The 

 change of color generally begins on the lower part of the sides in the assumption of a more 

 yellowish tint, which gradually extends towards the middle of the back, the top of the head 

 and nape being the last to change. The extreme edge of the ear is usually whitish, more 

 conspicuous than in the adult. 



The adult colors do not appear to be assumed for some time, perhaps several months or more. 

 I cannot establish any particular relationship between color and season, all the shades being 

 found in every month. 



I have taken the white-footed mouse of Massachusetts as my type in describing the species on 

 account of the very large series of specimens on hand received from Mr. Jenks, and repre 

 senting all the variations of age, sex and season. In comparing with these some skins from 

 Nova Scotia, probably identical with the Labrador species, called Hesperomys maniculatus by 

 Wagner, I find little, if any, appreciable difference. They appear rather smaller, the ears, 

 perhaps, a little larger, and the upper parts of the young have a grayish cast instead of a 

 plumbeous, somewhat as in the Upper Missouri species. The tail is, however, long, as in 

 H. leucopus, and the feet of the same character. 



On examining the extensive table of measurements of the white-footed mouse it will be seen 

 that, as a general rule, the tails are always longer than the body, exclusive of the head, 

 generally a little less than the total length, in a very few instances about one-third of an inch 

 longer. The length of the hind foot also varies from .75 to .85, never less than the former in 

 full grown specimens ; the more usual length is .80. 



Notwithstanding the similarity in external form between the white-footed mouse and the 

 house mouse, there are certain points of difference externally, in addition to the generic ones 

 derived from the skull and teeth. Thus the ears and eyes are considerably larger, the head 

 broader and more pointed, the feet and their toes longer, and the tubercles much larger ; the 

 heel is hairy, instead of perfectly naked ; the tail is more hairy ; the strong contrast of the 

 white belly and feet and under surface of the tail are never seen in the house mouse. 



The resemblance between Mm sylvaticus of Europe and the present species is quite close, 

 although a simple examination of the skull and teeth will at once settle the question. The 

 M. sylvaticus, in some of its varieties, is considerably larger, though not usually so ; and, as 

 Audubon and Bachman very justly state, the colors are much less vivid, the tail and ears 

 shorter, the white of the under surface does not extend so high on the sides, and there is a 

 yellowish spot, edged with dusky on the breast, never seen in H. leucopus. 



This species, for a long time, was considered to be the same as Mus sylvaticus of Europe, or 

 at best but a local variety ; as is now well known, however, they are not even of the same 

 genus. 



The first applicable name is that of Rafinesque, and although nearly all the species published 



