Baculum: Stalk broad, greatest length (varying with subspecies from 2.3 to 3.1 mm.) 1-1/2 to 1-3/4 times greatest breadth, 3-1/3 to 4-1/3 times greatest depth; three ossified processes, median one largest, more than twice as wide and as deep as shorter, slenderer, lateral processes; median process laterally compressed distally except in one specimen in which moderately inflated distally, proximally enlarged in some specimens (Fig. 21) and 1/3 to 2/5 length of stalk; base broad, posterior profile in dorsal view evenly convex throughout, at widest point of stalk abruptly incurved; basal tuberosities moderately to strongly developed, medially confluent; in end-view base wider ventrally than dorsally, dorsal concavity slightly to much deeper than the nearly flattened ventral concavity; medial constriction 2/3 to 4/5 of greatest depth; shaft relatively slender, at mid-point of stalk slightly wider than high and 1/4 as wide as base of stalk, terminally rounded or slightly inflated; lateral profile in dorsal view a gradual curve from point of greatest width anteriorly onto shaft. The different subspecies figured show the essential characteristics of the species, differing primarily in size. _Specimens examined_: Fourteen, of three subspecies; _Microtus montanus amosus_, 1/2 mi. E Soldier Summit, Wasatch Co., Utah, 62241; _M. montanus fusus_, La Manga Pass, Conejos Co., Colorado, 42164; 5 mi. N, 26 mi. W Saguache, 9500 ft., Saguache Co., Colorado, 42307, 42315; 5 mi. N, 27 mi. W Saguache, 9350 ft., Saguache Co., Colorado, 42308; 5 mi. N, 28 mi. W Saguache, 9325 ft., Saguache Co., Colorado, 42309; 5 mi. S, 24 mi. W Antonito, 9600 ft., Conejos Co., Colorado, 42327, 42330; Prater Canyon, Mesa Verde National Park, Montezuma Co., Colorado, 69456, 69457, 69463; _Microtus montanus nanus_, 2 mi. N, 2 mi. W Pocatello, Bannock Co., Idaho, 57470, 57472; 3/4 mi. N, 2 mi. W Allenspark, 8400 ft., Boulder Co., Colorado, 50330. Microtus townsendii (Bachman) Fig. 41 Baculum: Stalk broad, greatest length (3.0 mm.) 1-1/2 times greatest breadth, 4-1/2 times greatest depth; three ossified processes, median one largest, deeper and more than twice as wide as curved, shorter, compressed lateral processes and more than 2/5 as long as stalk; base broad, in dorsal view posterior profile trilobate, basal tuberosities visible; basal tuberosities well developed, medially confluent; in end-view base wider ventrally than dorsally, dorsal concavity deeper than ventral concavity; medial constriction 3/5 of greatest depth; shaft broad, at mid-point more than twice as wide as high and 1/3 as wide as base of stalk, terminally rounded. _Specimens examined_: Three, all _M. t. townsendii_; Fort Lewis, Pierce Co., Washington, 57998, subadult; Sec. 33, T. 11S, R. 5W, Benton Co., Oregon, 79186; Sec. 5, T. 12S, R. 4W, Benton Co., Oregon, 79188. Microtus oeconomus (Pallas) Fig. 44