MURID^— ARVICOLIN^— ARVICOLA RIPARIUS. 



183 



darker than the other, iVA; but either can be matched in color by both adult 

 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 Beeslcy'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 nf 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, III., 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 wbfbh 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 

 iuto 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 Audnbon and 



