188 MONOGRAPHS OF NORTH AMERICAN KODENTIA. 



No. 988 of riparius, from Racine, Wis., above mentioned. The incisors are 

 yellow, us 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. 



Skull (No. y fii')- — There is nothing diagnostic in the skull of this 

 species, except its size, as compared with that of riparius. It measures, in 

 length, 1.27, 0.71 in width of zygomata, 48 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 riparius. 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. tHH 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 riparius. There is nothing to detain us in the 

 under series. 



