Microtus richardsoni (DeKay, 1842) . Zoology of New York, Part I, Mammals, p. 91.

TYPE LOCALITY: Canada, Alberta Prov., vicinity of Jasper House (as interpreted by Bailey, 1900:60) .

DISTRIBUTION: Subalpine and alpine meadows of Rocky Mountains, from S British Columbia and Alberta, Canada, to W Wyoming and C Utah, USA; and of Cascade Mountains, from SW British Columbia south through WC Oregon.

SYNONYMS: arvicoloides, macropus, myllodontus, principalis .

COMMENTS: Senior synonym of the type (= arvicoloides) of Aulacomys . In early classifications (Bailey, 1900; Miller, 1896), richardsoni was placed with Old World water voles of the subgenus Arvicola (including Aulacomys) of Microtus, a relationship reaffirmed by Hooper and Hart (1962) and followed by others, with Arvicola employed either at the subgeneric (Hall, 1981) or generic level (Jones et al., 1975). Other evidence favors the retention of richardsoni within Microtus and the restriction of Arvicola to Old World forms (Carleton, 1981; Gromov and Polyakov, 1977; Hinton, 1926; Koenigswald, 1980; Repenning, 1980; Zagorodnyuk, 1990). Hoffmann and Koeppel (1985) further suggested the de novo origin of M. richardsoni in North America from an early stock that also gave rise to M. xanthognathus, an idea consistent with the biochemical similarity of richardsoni to certain New World Microtus (Nadler et al., 1978) and with Zagorodnyuk's (1990) expanded concept of the subgenus Aulacomys (including M. chrotorrhinus, M. longicaudus, and M. xanthognathus). The phylogenetic placement of richardsoni in Arvicola versus Microtus bears critically on interpretations of historical zoogeography, but the appropriate sampling of New and Old World species needed to resolve the problem has yet to be achieved within a single study. See Ludwig (1984, Mammalian Species, 223).