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 a pair. 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 vibrissso 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 

 -13 



