MUIJIDiE— ARVICOLIN^E— ABVTCOLA XANTDOGNATHUS. 199 



The under parts are uniformly strong hoary-ash, owing to whitish tips of the 

 very deep plumbeous fur. The hoariness varies much, sometimes being 

 scarcely evident, and at other times being soiled with a brownish or clay- 

 colored wash. The very edges of the lips are apt to be whitish; the 

 whiskers are whitish and dark; the incisors yellow, the upper generally 

 deepest-tinted, the under frequently nearly white. 



We will finish consideration of color before taking up the matter of the 

 measurements. Our remarks are based upon an examination of several 

 hundred specimens. 



The range of individual variation appears to us comparatively slight. 

 This is probably due to the rather homogeneous character of the localities 

 whence our specimens came. Some are a little darker, or a little lighter, 

 browner, grayer, more grizzly, &c, than others. But we see nothing that 

 calls for special remark in this connection. One (No. 6594) is an incomplete 

 albino, having a broad zone of pure white around the head and neck, thence 

 extending alon<? the breast and bellv, and flesh-colored ears. 



Certain seasonal conditions of pelage are strongly marked. In the 

 spring, just after the animal has shed its old winter coat, it appears in a fur 

 very noticeably shorter, finer, smoother, and glossier than it is at other 

 seasons; and the color is so different as to readily suggest specific distinction 

 to one ignorant of the facts in the case. At a little distance, the animal 

 looks almost black, so dark is the shade. The brown of the upper parts is 

 almost a blackish mahogany-color, with little or no admixture of fulvous, 

 tawny, or yellowish-brown, while the plumbeous below has corresponding 

 intensity and is but slightly hoary. The tail is nearly unicolor. As the season 

 advances, the depth of color insensibly lessens ; the grayest and grizzli- 

 est animals are those that wear the oldest pelage in the fall and winter. 

 But we do not notice, in these or other changes, any marked difference in 

 the cheeks, the distinctive brand of the species usually remaining much 

 the same. 



On the other hand, individual variations in size are as great as we have 

 succeeded in demonstrating for riparius with our immense array of specimens 

 from all localities; and this is the more remarkable, seeing that xanthognathus 

 is so restricted in its geographical distribution that climatological influences 

 are hardly brought to bear upon it. We invite particular attention to the 

 subjoined table that demonstrates the variability we continually insist upon 



