In the absence of ossified lateral processes my two specimens differ from bacula of Microtus (Arvicola) terrestris figured by Didier (1943:79, 1954:245, 247, 248) and by Ognev (1950:591). The median process relative to the size of the shaft is smaller, and the shaft relative to its length is wider in M. richardsoni than in M. terrestris. The stalk of M. (Arvicola) amphibius figured by Didier is like that of M. richardsoni in its greater breadth and median notch on posterior border.
The relationship of the New World water rat, M. richardsoni, to the Old World water rats (genus Arvicola of some European authors) is uncertain. Miller (1896:66) placed all of them in the subgenus Arvicola. Subsequent authors, stressing differences in the teeth, have placed M. richardsoni in the subgenus Aulacomys of Rhoads. Zimmerman (1955) has shown that teeth in some Arvicola approach the more complex pattern of M. richardsoni. He argues also that Arvicola is generically distinct from Microtus on the grounds that the two groups have separate origins, Arvicola having descended from the genus Mimomys and Microtus from some other group of microtines. This argument also was advanced by Hinton (1926:47-48). Pending further studies of the possible polyphyletic origin of other subgenera of the genus Microtus, I refer both M. richardsoni and M. terrestris to the subgenus Arvicola.
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.).
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.