Microtus breweri (Baird, 1858) . Mammalia, in Repts. U.S. Expl. Surv., 8(1): 525.

TYPE LOCALITY: USA, Massachusetts, Muskeget Island, off Nantucket .

DISTRIBUTION: Known only from the type locality.

STATUS: IUCN - Rare.

COMMENTS: Subgenus Mynomes, pennsylvanicus species group sensu Zagorodnyuk (1990) . An insular vicariant of M. pennsylvanicus, the two are inseparable karyotypically (Fivush et al., 1975; Modi, 1986), marginally distinct electrophoretically (Kohn and Tamarin, 1978), but morphologically sharply discrete (Bailey, 1900; Miller, 1896; Moyer et al., 1988). Although posited as conspecific with M. pennsylvanicus (e.g., Corbet and Hill, 1991; Jones et al., 1986; Modi, 1986), Moyer et al. (1988) mustered convincing evidence for the retention of breweri as a species. See Tamarin and Kunz (1974, Mammalian Species, 45).