_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. [Illustration: 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.] SYSTEMATIC ACCOUNT OF SPECIES AND SUBSPECIES =Baiomys musculus= Southern Pygmy Mouse (Synonymy under subspecies) _Type._--_Sitomys musculus_ Merriam, Proc. Biol. Soc. Washington, 7:170, September 29, 1892. _Range._--Southern Nayarit, Michoacan, Mexico, Morelos, Puebla, and central Veracruz, southeastward to western Nicaragua, but unknown from southern Veracruz, Tabasco, and the Yucatan Peninsula (see Figure 10); occurs principally in the arid upper and lower divisions of the Tropical Life-zone. _Characters for ready recognition._--Unless otherwise noted, characters are usable only for the two age-categories of adult and old adult. Differs from _B. taylori_ in: hind foot 16 millimeters or more; occipitonasal length, 19 millimeters or more; zygomatic breadth, 10 millimeters or more; rostrum not deflected ventrally at frontoparietal suture but, instead, curving gradually toward anteriormost point of nasals; cingular ridges and secondary cusps on teeth more pronounced; basihyal having anterior pointed entoglossal process, shoulders of basihyal protruding anteriorly (characteristic of all age categories); baculum having broader shaft, spatulate to knob-shaped tip, wings at base projecting anteriorly; baculum more than 3 millimeters long; short process of incus knob-shaped rather than attenuate; muscular process of posterior crus of stapes prominent.