Microtus (Chionomys) longicaudus (Merriam)
Fig. 48
Baculum: Base of stalk well developed, greatest length (3 mm.) 1¾ times greatest breadth, 32/3 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 ½ 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 ¼ 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.