-Female, adult, skull and skin, No. 34004, Univ. Kansas,
Mus. Nat. Hist.; from 6 mi. E Hamilton, 3700 ft., Ravalli County,
Montana; obtained on August 14, 1949, by John A. White; original
number 477.

_Geographic distribution._--Flathead and Bitterroot valleys of
western Montana.

_Diagnosis._--Size small for the species (see measurements). Color:
Essentially as in _Microtus montanus nanus_. Skull: Small, slender,
and comparatively smooth; rostrum moderately depressed distally;
nasals moderately inflated distally and extending posteriorly not
quite to tips of premaxillary tongues; nasals usually truncate
posteriorly, but rounded in some individuals; premaxillary tongues
terminating posteriorly in a short medial spine; zygomatic arches
lightly constructed and usually more widely spreading posteriorly
than anteriorly; interparietal comparatively long and terminating
in a small, but distinct, medial spine, otherwise approximately
rectangular in shape; exposed parts of upper incisors short and,
for the species, only slightly procumbent; molar dentition weak
and, in most specimens, especially so posteriorly; tympanic bullae
large and well inflated, especially ventrolaterally; basioccipital
narrow owing to the encroachment of the tympanic bullae.

_Comparison._--Among named forms, _Microtus montanus pratincolus_
most closely resembles _Microtus montanus nanus_. The geographic
range of _M. m. nanus_ adjoins that of _M. m. pratincolus_ on three
sides; there is no conspecific subspecies adjoining the range of
_M. m. pratincolus_ on the north. From _M. m. nanus_, _M. m.
pratincolus_ differs as follows (measurements are all of males,
those of _M. m. nanus_ being of nine topotypes and near topotypes
from central Idaho): size smaller (149 mm. as opposed to 165), tail
shorter (37 as opposed to 39), hind foot shorter (19 as opposed to
20), upper molar series shorter (expressed as a percentage of
basilar length, 25.5 as opposed to 26.3), mastoidal region broader
(expressed as a percentage of basilar length, 48.6 as opposed to
46.7), braincase slightly more vaulted (depth of braincase
expressed as a percentage of basilar length, 31.3 as opposed to
30.4) and more inflated laterally; tympanic bullae more inflated,
this inflation being the most conspicuous diff