MUlllDiE— ABVIOOLIN/E-AKOTIC AUVICOL.E. 



189 



Wl»en we bp^iin to look up A. towmendi, wo anlici|mlc(l no (lifficulty in 

 mnkin^ it out 8|K>ci(ically iliNtinct from ordinary riparius; \nii even witii v<>ry 

 little nmterial to work up, wc nrc fairly drawn to the op|>oHite conclusion. 

 In fact, the only tnngihlc diiferencc we ran make out '\» that towntcndii ia 

 larger, witii a longer tail on an average; and even this is not constant, for 

 sevenil, out of our few specimens, exceed average rij)ariu,\ but little, or not 

 at all, and come well within the limits of riparius. Still these specimens 

 might have grown a little larger, and the average of the scries stand at the 

 extreme limit of ripariwt, while their tails and cars exceed this limit. Wo 

 therefore feel justified in retaining the name tmonsendi as expressive of a 

 geographical difTerentiation, occurring in a particular locality, to the apparent 

 exclusion of the usual United States exponent of the subgenus. It is not at 

 all a reliable species, and one of its strongest fcatures-^lcngth of tail — 

 reminds us forcibly of the case of Uesperomi/s "boylii" from the same local- 

 ities; while the Columbia River scries atTords direct passage into riparius. 



We have taken some pains to inform ourselves respecting tlie variation 

 in size of the common large Euro|)ean species of the genus Arvicolti amphi- 

 bim, the well-known water-rat; and wc find that all the variability we claim 

 for rijiariui, and even for the Arctic forms of that species (as we shall pres- 

 ently see), sinks into insignificance beside the variations known to, and 

 admitted by, all the better-informed writers in the case of A. amphihius. 



The propriety of sinking A. townaendii to u mere variety of riparius 

 will appear in still stronger light when we have dealt with Arctic animals of 

 this subgenus full seven inches long. We tiiko up this question next. 



Discussion of the Arctic AsvicoLiE of the bipabius type. 



The United States variations of A. riparius are disposed of easily in 

 com)>ari8on with the intricacies of the Arctic material, respecting which some 

 general observations will be offered. 



Although we have handled a far larger number of these animals than 

 has ever before been examined by all other investigators put together, yet, 

 perhaps for the very reason that we have seen so many specimens, we are 

 still unprepared to make identifications without reserve. Nor can we deter- 

 mine with certainty all of Richardson's supposed species, afler careful study 

 of his accounts, as well ns of Audubon's and Bachman's supplementary 

 notices, and although we have been favored by Dr. Sclater with additional 



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