The evidence afforded by the bacula available is not conclusive as to relations of Old World and New World water rats. No general agreement on the number of species in this Palaearctic group has been reached, and bacula of only three or four of the numerous Old World subspecies have been figured. I have examined none.
_Specimens examined_: Two, from Wyoming; 42454 (31 mi. N Pinedale, 8025 ft., Sublette Co.), 37903 (23-1/2 mi. S, 5 mi. W Lander, 8600 ft., Fremont Co.).
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.