Baculum: Stalk broad, greatest length (3.7 to 4.3 mm.) 1-1/2 times greatest breadth, relatively flattened, greatest depth 1/3 greatest breadth; single median ossified process, in smaller of two specimens this ossification incomplete and of unusual shape (Fig. 39); length of stalk 4 times length of median process; concavities of basal tuberosities medially confluent, constriction less than 1/2 greatest depth; widest point of shaft less than 1/4 length of shaft from posteriormost point; shaft wider than high except at distal end that is inflated dorsally and sometimes laterally; both ventral and dorsal concavities of base of stalk broad and moderately deep; posterior profile in dorsal view evenly rounded or having marginal notch. 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