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.