Microtus (Chionomys) nivalis Martins
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.