_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 when 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