On the basis of shape and size of auditory ossicles, Baiomys resembles South American hesperomines (Calomys and Thaptomys) rather than North American hesperomines.
1894. Baiomys True, Proc. U. S. Nat. Mus., 16:758, February 7. Type, Hesperomys (Vesperimus) taylori Thomas.
Diagnosis.—Size small (total length in adults, 93-135); tail shorter than head and body; hind foot in adults 12-17; ears small (8-12) and rounded; upper parts blackish sepia to ochraceous-buff; underparts slaty gray to white or pale buffy; eyes small; hind feet having six plantar pads, soles nearly naked except for some hairs on anterior parts of soles and anteriorly to base of toes and between toes; occipitonasal length of skull in adults, 17.0-21.5; zygomatic breadth, 9.0-11.5; coronoid process of mandible well developed, strongly recurved; ascending ramus of mandible short and erect; anterior palatine foramina (incisive foramina) long, usually terminating posterior to plane of the front of first molars; posterior palatine foramina nearly opposite middle of M2; interorbital space wide relative to widest part of frontals; nasals projecting only slightly over incisors; condyle terminal; upper incisors relatively heavy; primary first fold of M3 obliterated at an early stage of wear; major cusps of upper and lower anteriormost two molars alternating, more so in m1-m2 than in M1-M2, dental formula I/i, 1/1; C/c, 0/0; P/p, M/m, 3/3 = 16.
For distribution of the genus, see [Figure 9].
Fig. 9. Geographic distribution of the genus Baiomys. Black area shows where the two species occur together. Black dot (Acultzingo, Veracruz) shows locality where Baiomys taylori occurs within the range of B. musculus, but B. musculus is not known to occur at that locality.