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.

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.) 2¹/₃ 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.) 1½ times greatest breadth, 3½ times greatest depth; three digitate processes ossified; median process relatively broad, in length more than ½ length of stalk, triangular in dorsal view, with small spurs posterolaterally, middorsal ridge posteriorly; lateral ossifications slightly curved, slenderer, less than ½ depth and less than ½ 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 ½ 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.

Microtus guentheri Danford and Alston

Fig. 23

Baculum: In the one specimen examined, stalk broad, greatest length (2.9 mm.) 1½ times greatest breadth, 3½ times greatest depth; three digitate processes ossified; median process slightly less than ½ length of stalk, broad, dorsally curved; curved lateral ossifications shorter and more slender than median ossification; basal tuberosities well developed, angular, confluent across posterior border of projecting shelf; in end-view tuberosities projecting ventrolaterally from central shelf; dorsal surface at medial constriction flat, ventral surface broadly and deeply concave; posterior profile in dorsal view trilobate, central lobe formed by posteriorly flattened shelf, surface of attachment visible only on lateral lobes; at mid-point stalk almost twice as wide as deep, depth of shaft greater than width proximal to inflated terminus.

Specimen examined: One from Palestine, 67104.

Microtus fortis Büchner

Fig. 25

Baculum: Stalk large, greatest length (3.8 mm.) 1⁴/₅ times greatest breadth, 4½ times greatest depth; three digitate processes ossified; median ossification almost ¹/₃ length of stalk; lateral ossifications slender, smaller than median ossification; posterior profile of stalk in dorsal view trilobate, basal tuberosities well developed, confluent medially; in end-view dorsal