Part 3 (2/2)

Baculum: Greatest length of stalk (2.7 mm.) 2-1/4 times greatest breadth, 4-1/2 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 1/2 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.) 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.

Microtus arvalis (Pallas)

Fig. 22

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.

From the baculum of _Microtus arvalis_ figured by Ognev (1950:173), and from the baculum figured by Didier (1954:238) my specimen differs in the absence of lateral ossifications in the digitate processes, smaller and slenderer median ossification, and weaker base. These differences in part may be owing to a difference in age, my specimen being the less mature.

_Specimen examined_: One from Vidy, Switzerland, 67101.

Microtus orcadensis Millais

Fig. 24

Baculum: In the one specimen examined, stalk broad, greatest length (2.6 mm.) 1-1/2 times greatest breadth, 3-1/2 times greatest depth; three digitate processes ossified; median process relatively broad, in length more than 1/2 length of stalk, triangular in dorsal view, with small spurs posterolaterally, middorsal ridge posteriorly; lateral ossifications slightly curved, slenderer, less than 1/2 depth and less than 1/2 transverse thickness of median process; basal tuberosities well-developed, confluent medially; in end-view base wider dorsally than ventrally, dorsal concavity broader and more abruptly curved at mid-point than ventral concavity; constriction 1/2 greatest depth; posterior profile in dorsal view notched, setting off a posterior shelf; stalk including shaft wider than deep throughout, at mid-point width twice depth; lateral profile abruptly curved anterior to point of greatest width, sides of shaft tapering gradually anteriorly to rounded uninflated tip.

The baculum of this insular species, placed in the ”_arvalis_” group by Ellerman (1941:595), resembles the baculum of both _Microtus agrestis_ and _Microtus guentheri_ more than it resembles the baculum of _Microtus arvalis_. Similarities in the chromosomes of _M. arvalis_ and _M.

orcadensis_ were noted by Matthey (1953:254, 279), who was of the opinion that _M. orcadensis_ is an insular derivative of the _arvalis_-group.

_Specimen examined_: One from the Orkney Islands, 67106.

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