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.
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.
Baculum: In the one specimen examined, stalk broad, greatest length (2.9 mm.) 1-1/2 times greatest breadth, 3-1/2 times greatest depth; three digitate processes ossified; median process slightly less than 1/2 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.