Microtus (Chionomys) longicaudus (Merriam)
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.
Baculum: In the single specimen examined, stalk small, greatest length (2.3 mm.) 2-1/3 times greatest width, almost 6 times greatest depth, flattened proximally; three well-developed digitate processes, the median one ossified, the lateral processes cartilaginous; median ossification laterally compressed and decurved at tip, bilobate at base; basal tuberosities of stalk weakly developed, medially confluent; posterior profile in dorsal view evenly rounded; ventral concavity deeper and narrower than dorsal concavity, but both comparatively shallow; medial constriction 2/3 greatest depth; shaft straight, at mid-point twice as wide as deep; lateral profile tapering from greatest width gradually to parallel sides of distal third of stalk.