The mid-ventral white stripe characteristic of _pullus_ is present in three of 28 adults from El Salvador. Goodwin (1942:160) reported white hairs on the pectoral region of several topotypes of _B. m. grisescens_. The areas of white hairs on the venter of _grisescens_ occur in approximately 10 per cent of the specimens examined, whereas in _pullus_, the frequency of occurrence is 90 per cent. The areas of white hairs in _grisescens_ are in broad patches on the pectoral region, while in _pullus_, a white stripe passes from the pectoral region to the inguinal region in both males and females. I know of no selective advantage that the presence of this white stripe would confer on the mice. _Specimens examined._--Total 46, all from NICARAGUA, and distributed as follows: Type locality, 32 (including the type); _9 mi. NNW Esteli_, 8; _8 mi. NNW Esteli_, 3; San Rafael Del Norte, 1 (Amer. Mus. Nat. Hist.); _1 mi. NW Jinotega_, 1; Matagalpa, 1 (Amer. Mus. Nat. Hist.). _Marginal records._--NICARAGUA: San Rafael Del Norte; Matagalpa; type locality. =Baiomys taylori= Northern Pygmy Mouse (Synonymy under subspecies) _Type._--_Hesperomys_ (_Vesperimus_) _taylori_ Thomas, Ann. Mag. Nat. Hist., Ser. 5, 19:66, January, 1887. _Range._--Southeastern Arizona and southwestern New Mexico, south into Chihuahua and Durango, just east of the Sierra Madre Occidental, thence southeast through Zacatecas, Aquascalientes, Jalisco, Queretaro, and Guanajuato; two fingerlike projections extend northward, one on the west along the coast of Sinaloa into southern Sonora, and the other on the east covering eastern San Luis Potosi, Tamaulipas, eastern Coahuila, Nuevo Leon, into south, southeast, and north-central Texas. Southern margin of range in central Mexico approximates the 19th degree of latitude (see Figure 11). Arid lower and arid upper subdivisions of the Tropical Life-zone in south; principally Lower Sonoran and Lower Austral life-zones in north. _Characters for ready recognition._--Unless otherwise noted, characters are usable for the age-categories of adult and old adult. Differs from _B. musculus_ in: hind foot less than 16 millimeters; occipitonasal length less than 19 millimeters; zygomatic breadth less than 10 millimeters; rostrum deflected ventrally at frontoparietal suture rather than curving gradually toward anteriormost point of nasals; cingular ridges and secondary cusps on teeth reduced or absent; basihyal having entoglossal process much reduced or absent, shoulders of basihyal not protruding anteriorly, but more flattened (characteristic of all age categories); baculum having narrower shaft, knob-shaped tip, wings at base projecting laterally, baculum less than 3 millimeters long; short process of incus attenuate; muscular process of posterior crus of stapes reduced.