MURIDA—ARVICOLINA—ARVICOLA RIPARIUS. 183 
and young examples from other localities. 
The third line of variation, into unusual darkness, is fully illustrated in 
the series. The darkest Atlantic example is a very large one from Beesley’s 
Point, N. J. The muzzle, feet, and tail are almost black, and the latter is 
hardly appreciably paler underneath than on top, and has no line of demarka- 
tion whatever. The fur combines shortness, thickness, and great gloss with 
a coarse, bristly texture; the longer glossy hairs being so numerous as to 
recall the pelage of a muskrat. The other darkest specimens are the series 
from West Northfield, Ill., and Racine, Wis., and that from Fort Crook, Cal. 
In these, there is no, or no appreciable, rufescent shade, and the blackish- 
brown is considerably grizzled with gray. The West Northfield and Fort 
Crook examples are among the most closely matched of the whole series; abso- 
lutely no difference is to be observed. ~They present the very long, dense, 
and almost fluffy character of pelage upon which the variety “‘longipilis” was 
founded. The fur of Arvicolas, however, varies in these points so much, with 
age, health, season of the year, and climate, that ‘longipilis” cannot con- 
stitute even a permanent variety. Only a part of the Illinois series exhibits 
the feature, and it is represented solely by winter or early spring skins ‘The 
Fort Crook specimens, as just stated, are identical, and others on the Atlantic 
side approach to or recede from the character in indefinite and endless 
degree. 
The coloration of the tail, whether bicolor or not, is often introduced 
into specific diagnoses. In general, the tail above corresponds with the color 
of the back, whatever that may be, and below with that of the belly; and 
usually there is an obvious dividing line between the two colors. This line 
may be sharp, straight, and conspicuous, or completely wanting, when the 
color above shades insensibly into that below; and in either case the differ- 
ence between the two colors may be conspicuous or barely or not perceptible. 
In one specimen (No. 988, Racine, Wis.), the tail is of almost precisely the 
same dark-brown tint above and below, but has a large pencil of snow-white 
hairs at the tip. The condition of this pencil of hairs is extremely variable, 
as shown in the table of measurements; it varies from almost nothing to 
three or four lines in length. 
Although it is hardly necessary to discuss any of the names that have 
not been admitted since 1857, yet the Arvicola “nasuta” of Audubon and 
