n one of us (Hall, Proc. Biol. Soc. Washington, 51:131-134,
August 23, 1938) arranged several nominal species of _Microtus_ as
subspecies of the species _Microtus montanus_, _Microtus canicaudus_ was
not included because that writer had not examined representative
specimens. In the U.S. Biological Surveys collection in the U.S.
National Museum we have examined specimens of _M. m. canicaudus_, all
from Oregon, as follows: Hood River (Catalogue Nos. 262583-262586);
Canby (262577, 262578); Wapinitia (79985-79988); Sheridan (69779,
69780); McCoy (75834-75842, 77744); Salem (246736); Albany (161554); and
Corvallis (242552). The four specimens from Wapinitia seem to be those
that Bailey (N. Amer. Fauna, 17:29, June 6, 1900) listed as _Microtus
montanus_. The diagnostic characters mentioned by Miller in the
original description (Proc. Biol. Soc. Washington, 11:67, April 21,
1897) included the following: size approximately the same as in
_Microtus [montanus] nanus_; upper parts yellowish; tail usually nearly
uniform grayish above and below; auditory bullae much inflated; lateral
pits at posterior edge of bony palate unusually shallow. Because the
tails of the original series were understuffed and variously rotated,
they seemed to be less sharply bicolored than is the case, as shown by
subsequently collected specimens. Otherwise we find that the characters
mentioned above differentiate _canicaudus_ from its nearest relatives,
_Microtus montanus canescens_ to the northward, _M. m. nanus_ to the
eastward, and _M. m. montanus_ to the southward. In _canicaudus_ we have
noted one additional differential character; the interpterygoid space is
acuminate anteriorly. In this feature and in each of the other features
mentioned above, intergradation with _Microtus montanus nanus_ is seen
in the specimens from Hood River and Wapinitia. In the specimens from
Hood River the auditory bullae are only slightly less inflated than in
those topotypes of _canicaudus_ having the smallest bullae; there is
appreciable variation in size of the bullae in the topotypes. Even so,
the minimum size of bullae among the topotypes is larger than the
maximum size in the specimens from Wapinitia. The four specimens from
Wapinitia have the yellowish color of _canicaudus_ to a considerable
degree, and show intergradation between _canicaudus_ and _nanus_ in
depth of the palatal pits and shape of interpterygoid space. The
slightly larger size of these specimens from Wa