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.