Fig. 47
Baculum: Greatest length of stalk (2.7 mm.) 2¼ times greatest breadth, 4½ times greatest depth; three digitate processes, lateral processes mostly cartilaginous in single adult examined; median process well ossified, approximately 1/3 length of stalk, basally notched, not arched, laterally compressed distally; base of stalk broad and flat, basal tuberosities well developed, separate; posterior profile in dorsal view rounded, convex except for medial notch separating tuberosities; dorsal and ventral concavities deep, broad, equal; medial constriction less than ½ greatest depth; in dorsal view shaft tapering gradually from widest point, terminally rounded; at mid-point of stalk almost twice as wide as high.
In the elongate, largely cartilaginous lateral processes of the baculum, the specimen described above resembles M. longicaudus. The size of the median process in comparison to the size of the stalk is also the same. The lateral processes have larger ossifications and the base of the stalk is more robust in M. longicaudus than in M. nivalis.
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.) 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.