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. Microtus guentheri Danford and Alston Fig. 23