Baiomyini new tribe. Type genus—Baiomys True, 1894. Definition—small terrestrial muroid rodents with tail shorter than head-and-body (Hooper, 1972; Osgood, 1909; Packard, 1960); hindfoot narrow with digits I and V short relative to II-IV, plantar pads 1 and 4 proximally positioned compared with 2-3 (Carleton, 1980; Osgood, 1909); cranium small but stoutly built, rostrum short, interorbit and braincase lacking ridges; interparietal compressed, wedge-shaped, not contacting squamosal; carotid circulation derived (character state 3 per Carleton, 1980); tegmen tympani adnate to squamosal (Voss, 1993), alisphenoid strut present; M3 cylindrical, less than ½ size of M2; vertebral column with 13 thoracic and 6 lumbar vertebrae, 1^st rib articulating 7^th cervical and 1^st thoracic, humerus lacking entepicondylar foramen (Carleton, 1980); trochlear process of calcaneum distal (Carleton, 1980); two complete and five incomplete transverse palatal ridges (Carleton, 1980); stomach unilocular-hemiglandular, caecum a short simple sac, gall bladder present (Carleton, 1973, 1980); ampullary glands simple and preputial glands large (Arata, 1964; Carleton et al., 1975); baculum and phallus short and broad, with a shallow crater and terminal urinary meatus, dorsal and ventral lappets absent (Blair, 1942; Hooper, 1960); voice a staccato trill, males with prominent midventral glands (Blair, 1941; Hooper and Carleton, 1976). Contents—Baiomys True, 1894; Scotinomys Thomas, 1913.

Relegated to a subgenus of Peromyscus by Osgood (1909) but reinstated as a separate genus by Miller (1912b), a status reinforced by Blair (1942) and Hooper (1958). Relationship to South American forms (i.e., Calomys) advanced by Packard (1960), but other studies have disclosed cognate affinity to Scotinomys (Carleton, 1980; Carleton et al., 1975; D’Elía, 2003; Engel et al., 1998; Hooper, 1960; Hooper and Musser, 1964a). Revised by Packard (1960), who delineated subspecific ranges and also summarized fossil taxa (early Pliocene through Pleistocene) within a phylogenetic context. Karyology evaluated by Yates et al. (1979) and biochemical variation by Calhoun et al. (1989). Hershkovitz (1962) transferred the South American form hummelincki, erroneously described under Baiomys, to the phyllotine genus Calomys.