MURIDAS—ARVICOLIN—ARCTIC ARVICOLA. 189 
When we began to look up A. éownsendi, we anticipated no difficulty in 
making it out specifically distinct from ordinary riparius; but even with very 
little material to work up, we are fairly drawn to the opposite conclusion. 
In fact, the only tangible difference we can make out is that townsendii is 
larger, with a longer tail on an average; and even this is not constant, for 
several, out of our few specimens, exceed average riparius 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 series stand at the 
extreme limit of riparius, while their tails and ears exceed this limit. We 
therefore feel justified in retaining the name ‘ownsendi as expressive of a 
geographical differentiation, 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 features—length of tail— 
reminds us forcibly of the case of Hesperomys “boylii” from the same local- 
ities; while the Columbia River series affords direct passage into riparius, 
We have taken some pains to inform ourselves respecting the variation 
in size of the common large European species of the genus Arvicola amphi- 
bius, the well-known water-rat; and we find that all the variability we claim 
for riparius, 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. amphibius. 
The propriety of sinking A. townsendii to a 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 take up this question next. 
Discussion of the Arctic ARV1COLE of the RIPARIUS type. 
The United States variations of A. riparius are disposed of easily in 
comparison 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, after careful study 
of his accounts, as well as of Audubon’s and Bachman’s supplementary 
notices, and although we have been favored by Dr. Sclater with additional 
