Microtus umbrosus Merriam, 1898 . Proc. Biol. Soc. Washington, 12:107.

TYPE LOCALITY: Mexico, Oaxaca, Mt. Zempoaltepec, 8200 ft .

DISTRIBUTION: Known only from Mt. Zempoaltepec.

COMMENTS: Type species and sole member of Orthriomys, a taxon typically identified as a subgenus of Microtus (Anderson, 1959; Bailey, 1900; Hall and Cockrum, 1953; Zagorodnyuk, 1990). Martin (1974:61) suggested that this species "may eventually" be included in Neodon .