Baculum: Stalk broad, greatest length (2.2 mm.) 1-3/4 times greatest breadth, 3-1/2 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. Microtus (Stenocranius) gregalis (Pallas) Fig. 34 Baculum: Length of stalk (2.4 mm.) 1-3/4, times greatest breadth, 4-1/3 times greatest depth; median ossified process well developed, more than 1/3 length of stalk, higher than wide, slightly bowed, closely appressed to terminus of shaft; basal tuberosities of stalk moderately developed, confluent medially, posterior profile of medial apex rounded in dorsal view, lateral indentations present, hence trilobate outline; in proximal end-view base wider ventrally, ventral concavity broader than dorsal concavity but of equal depth, medial constriction 2/3 greatest depth; shaft slender in distal part, inflated terminally, and wider than high at mid-point of stalk; lateral profile a smooth slope of gradually decreasing curvature from point of greatest width to near distal end. The baculum of this species figured by Ognev (1950:461) differs in having lateral ossified processes, and a more rounded base of the stalk. Resemblance to the New World _Stenocranius_ is discussed below. _Specimen examined_: One from "Eastern Europe," 8059.