188 MONOGRAPHS OF NORTH AMERIGAN RODENTIA. 
No. 988 of riparius, from Racine, Wis., above mentioned. The incisors are 
yellow, as usual; the claws brownish-white. 
There is not. much difference in color in any of the specimens before 
us. No. 3177, however, has the under side of the tail decidedly whitish, 
with a pretty distinct dividing line; No. 2396, apparently a very old one, 
and the largest Arvicola we have seen from the United States, is noticeably 
darker than the rest, and more grizzly; the under parts are hoary-ash 
instead of ashy-white. 
1595 
Skull (No. *%e?)—There is nothing diagnostic in the skull of this 
species, except its size, as compared with that of 77parius. It measures, in 
length, 1.27, 0.71 in width of zygomata, 048 in height; the molar series is 
0.30 long; the upper incisors project 0.25, the under 0.45; the condyle of 
lower jaw is over 0.90 from their tip. These dimensions, although not those 
of the largest animal before us, exceed the maximum we have found for 
United States riparius. Another specimen, however, measures only 1.10 by 
0.67, thus coming within the limits of réparius. We should judge, however, 
that it would have grown something larger in time, since the evident sutures, 
&c., are not those of an old animal. 
The dentition conforms strictly to the riparius type. The back upper 
molar shows the anterior spherical triangle and two exterior and one interior 
closed triangles; the latter much larger than either of the others. The 
posterior trefle or crescent differs appreciably on the right and left sides; but 
for either may be described as a long crescent, having the regularity of its 
convexity somewhat interrupted by bulging, simulating another external angle, 
and bearing upon the back part of its concavity a long curved spur that 
simulates a second little crescent lying on the belly of the first. The middle 
upper molar has an anterior triangle a little obliquely placed, then an external 
closed triangle, then an internal similar, and then a postero-external one. 
The front upper molar is exactly like the middle one, with an additional 
internal closed triangle. Thus there are altogether ten external angles in 
the series, counting the bulging upon the back of the crescent as one; and 
nine internal ones, counting the spur on the belly of the crescent. Both 
these counts exclude the back horn of the crescent, which is directly poste- 
rior. No. 1275 only differs slightly in the details of the crescent, in lacking 
the bulging upon its back. The back upper molar of another specimen is 
exactly like many samples of r/parius. There is nothing to detain us in the 
under series. 
