Isthmomys Hooper and Musser, 1964 . Occas. Pap. Mus. Zool., Univ. Michigan, 635:12.

TYPE SPECIES: Megadontomys flavidus Bangs, 1902 .

COMMENTS: Peromyscine. Species associated here were originally classified in Megadontomys, used either as a genus or as a subgenus of Peromyscus (Osgood, 1909) . Isthmomys was later diagnosed as a subgenus of Peromyscus (Hooper and Musser, 1964b) and maintained at this rank by Hooper (1968b); accorded generic status by Carleton (1980, 1989). Sister-group relationship with Megadontomys proposed by Carleton (1980) but unsupported by chromosomal banding data (Stangl and Baker, 1984b). Aspects of morphology considered by Carleton (1973, 1980), Hooper and Musser (1964b), and Linzey and Layne (1969, 1974); karyology by Stangl and Baker (1984b).