RODENTIA—MURINAE—HESPEROMYS LEUCOPUS. 
461 
back ; tbe 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 Resperomys 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 Mus 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 speeies published 
