Microtus (Arvicola) richardsoni (DeKay)
Figs. 38 and 39
Baculum: Stalk broad, greatest length (3.7 to 4.3 mm.) 1½ 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 ½ greatest depth; widest point of shaft less than ¼ 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.).