Phyllotis xanthopygus (Waterhouse, 1837) . Proc. Zool. Soc. Lond., 1837:28.

TYPE LOCALITY: Argentina, Santa Cruz Prov., coast of Santa Cruz .

DISTRIBUTION: NW Argentina (Catamarca) and C Chile (Atacama), south along both flanks of Andes to Santa Cruz Prov., Argentina, and adjacent Magallanes Prov., Chile.

SYNONYMS: oreigenus, vaccarum, wolffhuegeli .

COMMENTS: Viewed as a geographic race of P. darwini by Pearson (1958) and Hershkovitz (1962). Morphometric, electrophoretic, and karyotypic differentiation support the specific distinction of P. xanthopygus (Spotorno and Walker, 1983; Walker et al., 1984).