Peromyscus difficilis (J. A. Allen, 1891) . Bull. Am. Mus. Nat. Hist., 3:298.

TYPE LOCALITY: Mexico, Zacatecas, Sierra de Valparaiso .

DISTRIBUTION: W Chihuahua and SE Coahuila, south to C Oaxaca, Mexico.

SYNONYMS: amplus, felipensis, petricola, saxicola .

COMMENTS: Revised by Hoffmeister and de la Torre (1961) to include nasutus (see species account below) and comanche (transferred to P. truei — Schmidly, 1973b). Karyologic and biochemical evidence has suggested that P. difficilis and P. nasutus are sibling species (Avise et al., 1979; Zimmerman et al., 1975, 1978); classified as such by Carleton (1989), but see Janecek (1990). P. truei species group.