The well ossified lateral processes and enlarged base of Didier's (1954:240) specimen suggest that it is of a more mature individual than the one described above. These specimens of _M. nivalis_, as well as the specimens of _M. longicaudus_, exhibit dorso-ventral flattening of the mid-part of the base of the stalk. The baculum of a specimen from Switzerland is weakly developed, of small size (shaft 2.0 mm. in length), slender, thin, spatulate, and terminally inflated. Digital processes were not observed, perhaps owing to excessive maceration in preparation. The general appearance of the baculum is that of an immature individual, although the animal was not small (165 mm. total length in preservative). _Specimens examined_: Two _Microtus nivalis nivalis_; Zermatt, Valais, Switzerland, 67105; Wetterstein, Germany, 65127. Microtus (Chionomys) longicaudus (Merriam) Fig. 48 Baculum: Base of stalk well developed, greatest length (3 mm.) 1-3/4 times greatest breadth, 3-2/3 times greatest depth; three ossified processes; base of median process rounded; median process slightly curved in dorsoventral plane, in length almost 1/3 greatest length of stalk; ossifications in lateral processes variable in size, frequently widely separated from shaft by cartilage, rarely as large as median ossification; basal tuberosities usually well-developed, medially confluent; profile of base in dorsal view trilobate or irregularly convex throughout; constriction 1/2 greatest depth; shaft relatively straight or slightly bowed ventrally or dorsally, shaft at mid-point of stalk wider than high; tip of shaft laterally inflated; widest point of stalk approximately 1/4 length of stalk from proximal end; lateral profile in dorsal view tapers gradually onto shaft anteriorly from point of greatest width of stalk; shaft variable, from slender terminally and nearly parallel sided (Fig. 48), to broad distally and tapered. In many of the features that distinguish _M. longicaudus_ (and the closely related insular species _M. coronarius_) from other North American _Microtus_, _longicaudus_ resembles the Old World species of the subgenus _Chionomys_ (that is to say, _M. nivalis_, _M. gud_, and _M. roberti_). These features are medium size, long tail, grayish color, montane habitat, relatively short molar tooth-row, moderate sized and unconstricted incisive foramen, relatively decurved upper incisors, elongate nasals, relatively broad interorbital region without well-developed median ridge, and similar chromosomes (Matthey, 1955:178). For these reasons I am here referring _Microtus longicaudus_ to the subgenus _Chionomys_; previously it has not been referred to that subgenus. _Specimens examined_: Six, of three subspecies; _Microtus longicaudus littoralis_, Sullivan Island, Alaska, 42972, 42969; _M. l. mordax_, 3/4 mi. N, 2 mi. W Allenspark, 8400 ft., Boulder Co., Colorado, 50335, 76829; _M. l. sierrae_, Crane Flat, Mariposa Co., California, 50252, 50253.