MAMMALS OF MINNESOTA. 201 
The ear is 0.45-0.50 high, the lower incisor 0.30, upper incisor 
0.20. It is believed that there is normally. about the above- 
indicated difference between males and females of the same 
age and at the same place. The above were chosen because 
the two were taken under circumstances indicating that they 
were apair. In autumn the averages of mice collected would 
be quite unreliable, and it seems quite probable that more than 
a Single winter is necessary to the attainment of the full size. 
However, mice collected in spring about Minneapolis have 
proven very uniform in both size and color. A very large 
female, collected in October, 1883, measured about five inches, 
with a tail measuring 2.0, hind feet 0.75, and fore feet 0.43. 
The averages of 114 specimens of this species from east of 
the Mississippi, as given by Coues, are as follows: Trunk, 4.33; 
tail, 1.59; hind foot, 0.78. 
In our opinion the average of a large series of Minnesota 
specimens would be considerably less, and quite obviously so 
if the northern part of the state were represented. 
The color is dark brown with reddish sifted through the 
pelage. Usually the whole back from the nose to the tail is 
uniform dark brown, the upper surface of the tail being some- 
what darker, while the sides of the body are appreciably 
warmer. The upper surfaces of the feet are brown, also. 
Below the fur is ashy white, more whitish about the mouth. 
The lower surface of the tail is more or less distinctly white. 
Winter specimens are very uniform in color, and there is no 
sexual differentiation as in Fiber. The vibrissz are short and 
dark. 
The range of this species is very extensive yet it would 
seem that its true home is the eastern part of the United 
States from the northern part of the Gulf states northward to 
Labrador. Farther west the northern limit of typical Arvicola 
riparius seems to lie within the U. S. boundaries. Although 
the species extends westward to the Pacific it tends to lose its 
integrity and merges in varieties of greater or less permanence. 
Northwardly and westerly from the middle of Minnesota the 
size drops rapidly and in the Red River valley the measure- 
ments are never up to the average of the type. Still farther 
north it would seem that the species can not go without great 
difficulty or at the expense of much of its normal size. It here 
becomes variety borealis. It may be suggested that constant 
migrations (after the analogy of the migratory habit of the 
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