Microtus (Chilotus) oregoni (Bachman)
Fig. 45
Baculum: Stalk broad, greatest length (2.2 mm.) 1¾ times greatest breadth, 3½ times greatest depth; three well-developed ossified processes; median process 2/5 length of stalk, rounded or tapered terminally, proximal end opposed to tip of stalk and flattened obliquely; lateral processes 2/3 length of median process, deeper than wide, curved; tuberosities of stalk well developed, confluent medially, visible in dorsal view; in end-view dorsal concavity narrow, moderately deep, rounded, ventral concavity wide, deep, flattened; base wider ventrally than dorsally; shaft tapering more or less uniformly, terminally inflated.
In the relative sizes, to each other and to the stalk, of the three digitate ossifications M. oregoni resembles closely the Old World representative of the same subgenus, M. (Chilotus) socialis, as figured by Argyropulo (1933b:181). In M. oregoni the greatest width of the baculum is more proximal on the stalk than in the M. socialis figured by Argyropulo but closely resembles the baculum of the M. socialis figured by Didier (1954:242). In possessing a shallow emargination in the base of the stalk and in possessing a median process that is smaller than the lateral processes, M. socialis, as figured by Didier, differs from M. oregoni. The baculum figured by Argyropulo (loc. cit.) of Sumeriomys colchicus schidlovskii [ = Microtus (Chilotus) socialis schidlovskii according to Ognev, 1950:392] differs from other Chilotus that have been studied in having an unusually elongate median process and a more distal placement of the widest part of the stalk.
Specimens examined: Three, of the subspecies M. oregoni oregoni, from 5 mi. N Orick, Humboldt Co., California, 3-C-248, collection of W. B. Quay; from Mary's Peak, Benton Co., Oregon, 66, collection of F. W. Sturges; and from Sec. 3, T. 11S, R. 5W, Benton Co., Oregon, 79183.