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.

Microtus montanus canicaudus Miller

1897. _Microtus canicaudus_ Miller, Proc. Biol. Soc. Washington,
11:67, April 21, type from McCoy, Willamette Valley, Polk County,
Oregon.

In 1938 whe