Diagnosis.—Size medium to small for the species; general ground color of dorsum between Olive Brown and Buffy Brown; distal fourth of individual guard hairs of dorsum black-tipped, proximal three-fourths gray, underfur black-tipped with subterminal band of Vinaceous-Buff, gray basally; facial region below eye Olive-Buff to Deep Olive-Buff; regions of flanks without black-tipped guard hairs, therefore, appearing paler brownish-buff than dorsum; venter Pale Olive-Buff to whitish in midline, hairs there white to base, laterally grayish basally; hairs in region of throat and chin resemble those of underparts; forefeet and hind feet flesh-colored with grayish suffusion; ears dusky brown; tail almost unicolored, slightly darker brown above than below; coronoid process less acutely falcate than in other subspecies; zygoma bowed. Average and extreme external and cranial measurements of 14 adults from La Piedra de Jesús Sabana Grande, Honduras, are as follows: Total length, 110.7 (100-123); length of tail vertebrae, 44.0 (32-55); length of body, 66.7 (60-70); length of hind foot, 14.1 (12-15); length of ear from notch, 11.8 (10-13); occipitonasal length, 19.3 (18.9-19.8); zygomatic breadth, 10.1 (9.8-10.4); postpalatal length, 6.8 (6.2-7.3); least interorbital breadth, 3.9 (3.8-4.1); length of incisive foramina, 4.3 (4.0-4.5); length of rostrum, 6.9 (6.6-7.2); breadth of braincase, 9.6 (9.2-10.1); depth of cranium, 7.0 (6.8-7.3); alveolar length of maxillary tooth-row, 3.2 (3.0-3.4); for photographs of skull, see [Plate 1b], and [Plate 3b].
Comparisons.—For comparisons with B. m. pullus and B. m. handleyi, see accounts of those subspecies. From B. m. nigrescens, B. m. grisescens differs in: dorsum less blackish (dark brown to buffy); face buffy below eye rather than brownish-black; venter buffy to whitish in midline, not sooty gray; forefeet and hind feet flesh-colored with gray overtones, not dusky to sooty; zygoma bowed, sides less parallel; braincase and bony palate slightly broader.
Remarks.—Goodwin (1942:160) mentioned that a specimen from the type locality of grisescens was as dark as specimens of B. m. nigrescens from Guatemala. However, all specimens from Guatemala, other than those from Sacapulas, were referred by Goodwin (1934:40) to B. m. nigrescens. My studies reveal a grayish-brown population in central Honduras near to and including the type locality. This population appears to grade into a slightly paler, particularly as concerns color of hind foot and tail, group of Guatemalan mice from 1 mi. S Rabinal, from 1/2 mi. N, 1 mi. E Salama, and from Lake Atescatempa. Specimens from western Guatemala at Nentón and Jacaltenango, on the other hand, are darker brownish-black, more nearly like the paratypical series of nigrescens from the Valley of Comitán, Chiapas, Republic of México. This darker brownish-black color of the back persists in specimens from southern Guatemala and El Salvador (see specimens examined of B. m. nigrescens for localities), and they are best referred to nigrescens. B. m. grisescens, in color and certain cranial characters, therefore, seems to grade into two different subspecies: (1) B. m. handleyi, pale mice in the Río Negro valley in central Guatemala, and (2) B. m. nigrescens, dark mice from southern Guatemala, and parts of El Salvador.
Felten (1958:136) referred all B. musculus from El Salvador to B. m. grisescens. Although I have not examined the specimens reported on by Felten (loc. cit.), I have examined specimens from Lake Atescatempa, Guatemala (which I refer to grisescens), not too distant from Cerro Blanco, and Finca Las Canarias, Department of Ahuachapan, and Laguna de Guija, Department of Santa Ana (localities listed by Felten). It would seem that specimens from these localities might indeed be grisescens. However, specimens that I examined from 1 mi. S Los Planes, and 1 mi. NW San Salvador were considerably darker than paratypes of grisescens and were nearly intermediate in color between nigrescens and pullus. I refer the specimens from 1 mi. NW San Salvador, and 1 mi. S Los Planes to nigrescens rather than to grisescens.
There is no positive evidence that B. m. grisescens intergrades with B. m. pullus to the south in Nicaragua. But, there is a suggestion that intergradation occurs between these subspecies in a series of 76 skins from La Piedra de Jesús Sabana Grande, Honduras, referable to grisescens. A total of 16 of 76 skins from this locality (21 per cent) possess the mid-ventral white stripe found in 18 of 20 skins (90 per cent), from the type locality of pullus in Nicaragua. Further collection in areas between central Honduras and western Nicaragua may yield specimens of B. musculus that are intermediate in characters between grisescens and pullus.
Specimens examined.—Total 149, distributed as follows: Guatemala: 1 mi. S Rabinal, 3450 ft., 14; 1/2 mi. N, 1 mi. E Salama, 3200 ft., 10; Lake Atescatempa, 10[7]. Honduras: Cementario, Gracias, 1[8]; Monte Redondo, 1[8]; El Caliche, Cedros, 1[8]; La Flor Archaga, 2[8], 1[9]; Hatillo, 1[8]; type locality, 7[8], 6[7] (including the type), 3[9]; El Zapote, Sabana Grande, 4[8]; La Piedra de Jesús Sabana Grande, 76[8]; Cerro de las Cuches Sabana Grande, 5.
Marginal records.—Guatemala: 1/2 mi. N, 1 mi. E Salama, 3200 ft. Honduras: El Caliche, Cedros; Hatillo; La Piedra de Jesús Sabana Grande; Cementario. Guatemala: Lake Atescatempa; 1 mi. S Rabinal, 3450 ft.
[7] United States National Museum (Biol. Surv. Collections).
[8] American Museum of Natural History.