MURIDA—ARVICOLINAZ—ARCTIC ARVICOLZ. 193 
an animal as No. 4504 (beyond) with No. 9235 (beyond), and both of them 
with 77parius, 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 zan- 
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-pateh 
(an easy matter) and label them zxanthognathus, 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 zanthognathus 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 ‘‘riparius var.—?”. 
The distribution of zanthognathus 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 7iparius. The extreme 
of boreacis 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 réparius, not distinguishable in any way from that 
form, and not noticeably inclining to the characters of either var. borealis or 
var. zanthognathus. What slight discrepancies there are in average dimen- 
sions are noted at the conclusion of the table. 
13 M 
