330 ARVICOLID^E. 



The tail is about one-half the length of the body, and is 

 thickly clad with rather long hairs which form a tuft or 

 pencil at the end ; it is dark brown above and white 

 below, the two colours being clearly separated. 



In the dentition the most remarkable peculiarity, to 

 which we have already alluded, is the development in 

 the adult of distinct roots to the molar teeth. The 

 first upper grinder has four cemental spaces and five 

 angles, the second has six spaces and eight angles, and 

 the first lower grinder has seven spaces and nine angles. 



The following measurements in inches and decimals 

 are an average taken from a number of specimens of 

 both sexes : 



Males. Females. 

 Length of head and body . . . . 3 '50 3'40 



of head 1-10 1-08 



,, of ear -40 -40 



,, of tail 1-54 1-50 



,, of hind foot -62 '58 



This species is liable to considerable variation in colour 

 not only in individuals, but in the races which inhabit 

 different districts. Thus the Alpine form, in which the 

 grey tint predominates over the red, has been separated 

 as Hypud&us nageri of Schintz, and the opposite extreme 

 in which the grey is almost absent as Myodes bicolor of 

 Fatio. But these forms are not constant and pass one 

 into the other, as has been shown by Blasius and by 

 M. Fatio's later researches. 



