_Type._--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 difference between the
two subspecies. The tympanic bullae of _M. m. pratincolus_ have
approximately a fourth more volume than those of _M. m. nanus_.
_Remarks._--Northwardly in the Bitterroot Valley, specimens of _M. m.
pratincolus_ morphologically approach _M. m. nanus_, especially in the
reduced degree of inflation of the tympanic portion of the bullae. On
geographic grounds we think that the geographic range of _M. m.
pratincolus_ extends southward to the southern end of the Bitterroot
Valley; we have not seen specimens from that area. Although we have not
examined the specimen reported upon by Davis (Murrelet, 18:26, September
4, 1937) from Canyon Creek, "a few miles west of Hamilton", Montana, we
think that it will be found to belong to _M. m. pratincolus_.
Our examination of specimens from localities in Montana east of the
range here ascribed to _M. m. pratincolus_ indicates that, among named
kinds of _Microtus_, those specimens are best referred to _M. m. nanus_.
These specimens are listed below under comparative materials. It should
be mentioned here that although Bailey (_loc. cit._) applied the name
_Microtus nanus canescens_ to Montanan specimens from Flathead Lake and
Hot Springs Creek, the subspecies _M. montanus canescens_ now is thought
to be restricted to Washington and the adjoining part of British
Columbia; _M. m. canescens_ does not occur so far east as Montana.
Grateful acknowledgment is made to those persons in charge of the
Biological Surveys collection for permission to study the specimens in
that collection, and to the Kansas Endowment Association for support of
the field work which yielded the specimens from six miles east of
Hamilton, Montana. The study here reported upon was aided also by a
contract between the Office of Naval Research, Department of the Navy,
and the University of Kansas (NR 161-791).
_Measurements._--The following measurements in millimeters are
those of the holotype and the average, maximum, and minimum,
respectively, of eleven adult males from various places in the
range of the subspecies. Except as noted below, we are unable to
detect significant morphological differences in the populations
sampled and believe that pooling of the measurements is justifiable
in this case. Measurements are: Total length, 129, 149 (156-141);
length of tail-vertebrae, 27, 37 (39-31); length of hind foot, 18,
19 (20-18) (all preceding measurements are those of the
collectors); basilar length, 22.2, 24.5 (25.7-23.4); greatest
length of nasals, 6.7, 6.9 (7.4-6.4); zygomatic breadth, 14.2, 14.6
(14.9-13.9); mastoidal breadth, 11.3, 11.8 (12.3-10.8); alveolar
length of upper molar series, 6.0, 6.2 (6.5-5.9); depth of
braincase (shortest distance from ventral surface of
basioccipito-basisphenoidal suture to the dorsal surface of the
cranium, and not perpendicular to the long axis of the skull), 7.7,
7.7 (7.9-7.5); width of rostrum, 4.7, 4.8 (5.0-4.6); interorbital
breadth, 3.2, 3.4 (3.6-3.2). Measurements of females, other than
those of the holotype, are not given owing to the lack of
sufficient material. Females, however, do not appear to differ
appreciably in measurements from males.
_Specimens examined_ (in U.S. Nat. Mus., Biol. Surv. Coll., except
as otherwise indicated).--Total, 26, all from Montana, as follows:
_Sanders Co._: Hot-springs Cr., 4. _Lake Co._: end of W arm
Flathead Lake, 5; Ravalli, 8. _Ravalli Co._: Florence, 2; 8 mi. NE
Stevensville, 4000 ft., 1; Corvallis, 4; 6 mi. E Hamilton, 3700
ft., 2(K.U.).
_Comparative materials_ (in U.S. Nat. Mus., Biol. Surv.
Coll.).--_Microtus montanus nanus_: Total, 72, as follows: IDAHO:
_Lewis Co._: Nez Perce, 3. _Idaho Co._: Seven Devils Mts., 3550
ft., 3. _Custer Co._: Challis, 7; Pahsimeroi Mts. (9300 ft., 8;
9700 ft., 4), 12. _Butte Co._: Lost River Mts., 1. _Canyon Co._:
Nampa, 1; Bowmont, 1. _Ada Co._: Boise, 1. _Blaine Co._: Sawtooth
Lake, 2; Alturas Lake, 3. _Owyhee Co._: Three Cr., 3. _Minidoka
Co._: Heyburn, 2. _Bannock Co._: Pocatello, 4. _Bear Lake Co._:
Montpelier Cr., 3. MONTANA: _Fergus Co._: Big Snowy Mts., 11.
_Gallatin Co._: West Fork of West Fork, Gallatin River, 1. _Park
Co._: Lamar River, 7000 ft., 2; Gardiner, 1. _Sweetgrass Co._:
"near" head Big Timber Cr., Crazy Mts., 1; Big Timber Cr., 5200
ft., Crazy Mts., 1; 14 mi. S Big Timber, 1; West Boulder Cr., 18
mi. SE Livingston, 2; McLeod, 1. _Carbon Co._: Beartooth Mts., 2;
Beartooth Lake, 1. WYOMING: _Park Co._: N end Lake, Yellowstone
Nat'l Park, 2.