MOUID^E— ARYirOLlN^E— ARCTIC ARVICOLiE. 193 



an animal as No. 4504 (beyond) with No. 9235 (beyond), and both of them 

 with riparius, as not specifically distinct, ; yet we can find no other alternative, 

 since our series supplies every link in the chain. If we take dimensions, we 

 find every intermediate size, by tenths of an inch, from three to eight inches ; 

 it is the same with proportions of feet, tail, and ears, both relative and abso- 

 lute. Conditions of pelage are utterly confounding, even leaving season, if 

 not also latitude, out of consideration. Those characteristic examples of xan- 

 thognathus, in which the chestnut cheek is well marked, set aside, color gives 

 us nothing we can rely upon. If, therefore, there be more than one species 

 in the series, I must simply confess that I am not bright enough to discover 

 or define it. 



In dealing with this lot of material, I shall, in the first place, eliminate 

 the specimens not appreciably different in any respect from ordinary United 

 States riparius. I will then separate those that have the chestnut cheek-patch 

 (an easy matter) and label them xanthognathus, without reference to their 

 being a variety only of riparius. I shall be able to label many of the rest 

 var. borealis, and to indicate a considerable number as more or less nearly 

 approaching either xanthognathus or borealis. But a large residuum (includ- 

 ing most of the alcoholics, respecting which nicety of determination is obvi- 

 ously impossible) can be only marked as u riparius var. — !". 



The distribution of xanthognathus and borealis will be perceived from 

 the tables beyond. Southern and Central British American specimens, as a 

 rule, are more or less completely similar to ordinary riparius. The extreme 

 of borealis has only occurred, so far as we are aware, from the Arctic coast 

 and contiguous northwestern regions. Well-characterized xanthognathus 

 scatters over a larger area, but likewise focuses in the Northwest. It is a 

 further source of difficulty and doubt that these extremes are not geographi- 

 cally marked ; on the contrary, they occur side by side, and are, therefore, 

 not explicable upon the rules of geographical variation that we have elsewhere 

 laid down and somewhat successfully applied. This may be held as strong 

 evidence that these forms are specifically distinct ; but we must beg any one 

 who may so believe to show us any reliable specific character. 



The following table embraces a number of specimens more or less per- 

 fectly similar to ordinary riparius, not distinguishable in any way from that 

 form, and not noticeably inclining to the characters of either var. borealis or 

 var. xanthognathus. What slight discrepancies there are in average dimen- 

 sions are noted at the conclusion of the fable. 



13 M 



