MURIDH—ARVICOLINA—ARVICOLA RIPARIUS. 181 
The shade is slightly grizzled with dull yellowish-brown. Beneath, the plum- 
beous hairs are all tipped with white, resulting in a hoary-ash, which is clearest 
(whitest) on the breast and belly, darkest on the throat; there is no line of 
demarkation between upper and under parts. There is no cinnamon, tawny, 
nor muddy tinge underneath; but a faint brownish wash, like an extreme’ 
dilution of the color of the sides, is barely perceptible. 
This typical coloration prevails in the majority of eastern specimens in 
the present collection. Nearly all those from the Middle States, others from 
Nova Scotia, Massachusetts, the Carolinas, Wisconsin, &c., are not appreciably 
different. Other specimens show a departure from this standard in three 
courses: toward “red”, “black”, and “gray”. The extreme of the rufescent 
variation is reached in No. yyy, type of “rufidorsum”. In this, the color is 
an intense tawny above, a little darker on the rump, paler on the sides, and 
washing the under parts. The feet share the general tawny hue. This 
extreme case is not nearly matched by anything else in the collection, and 
raises a strong suspicion of an albinotic element. We know by the ‘“albo- 
rufescens” 
of Dr. Emmons that such a state does occur; although there is 
really a gap in the eastern series between “rufidorsum” and the rest, yet many 
specimens grade nearly up to it. In the Massachusetts series, where some 
(particularly winter and early spring skins) are quite as dark as any Philadel- 
phia ones, others are very much brighter. No. 7s, for example, is very 
rufescent, from predominence of bay in the: hairs. A Labrador specimen 
Tosa) is rather brighter still; No. 2661, from Nichols, N. Y., and a Pennsyl- 
vania skin, No. 4724, labeled “riparius” by LeConte, are quite as bright; so 
are a few of the Illinois and Missouri ones, and one from Louisiana (:%%0). 
But the link between the extreme of rufidorsum is afforded by No. 10083, the 
type of “occidentalis”, which might be described in identical terms; and, 
although the tawny is not quite so vivid, it similarly tinges the feet. 
The extreme of paleness or grayness is illustrated in what has been called 
“breweri”. In this, the upper parts are of a light dull grizzly-gray. with a 
small proportion of yellowish-brown, and the under parts soiled white; the 
hairs being only plumbeous at the extreme base, and consequently scarcely 
shading the whitish. The cause and nature of this variation have been so 
perfectly explained by Mr. Allen* that I shall quote his words:—‘‘On Mus- 
keget Island (a small, uninhabited, low sandy island between Nantucket and 
*Mammals of Massachusetts, in Bull. Mus. Comp, Zool. No. 8, p. 232. 
