_Diagnosis._--Size medium to large for the species; mid-dorsal region Clove Brown (sooty in freshly captured specimens); some parts of mid-dorsal region all blackish; individual guard hairs of dorsum black-tipped, Deep Neutral Gray basally; underfur black-tipped with subterminal band of light buff, Neutral Gray at base; belly grayish-white, laterally Isabella Color; distal three-fourths of hairs in region of throat and chin white, proximal fourth light gray; in median region of belly distal half of individual hairs white, proximal half dark gray; vibrissae in most specimens black to base. Average and extreme cranial measurements of six adults from 7 mi. S La Belle are as follows: occipitonasal length, 18.9 (17.5-19.4); zygomatic breadth, 9.6 (9.1-9.9); postpalatal length, 6.8 (6.2-7.2); least interorbital breadth, 3.7 (3.4-3.9); length of incisive foramina, 4.0 (3.6-4.2); length of rostrum, 6.5 (6.1-6.8); breadth of braincase, 8.7 (8.3-8.9); depth of cranium, 6.7 (6.6-6.8); alveolar length of maxillary tooth-row, 3.1 (2.9-3.2). Average and extreme external measurements of four adults from Richmond are as follows: total length, 111.5 (108-118); length of tail vertebrae, 43.5 (41-47); length of body, 68 (67-71); length of hind foot, 14 (13-15); for photographs of the skull, see Plate 2_f_, and Plate 4_g_. _Comparisons._--Because _B. t. subater_ intergrades only with _B. t. taylori_ to the south and west, _subater_ is compared only with _taylori_. Young adults of both subspecies in unworn pelage show best the colors that differentiate the two subspecies. Old adults of _subater_ in worn pelage appear grayish, resembling _taylori_, and at that age, only certain cranial characters are of taxonomic use. Cranially, _subater_ differs from _taylori_ in: presphenoid not shaped like an hour-glass; parapterygoid processes thicker medially; interparietal diamond-shaped instead of elongated and compressed. Skull slightly larger in most measurements. [Illustration: PLATE 1 Photographs of skulls in dorsal view of _Baiomys_. x 2. _a._ _B. m. brunneus_, [F] ad., 10834, AMNH, Jalapa, Veracruz. _b._ _B. m. grisescens_, [F] ad., 257080, USNM, Comayabuela, Honduras. _c._ _B. m. handleyi_, [F] ad., 275597, USNM, Sacapulas, Guatemala. _d._ _B. m. infernatis_, [F] ad., 91499, MZUM, Teotitlan, Oaxaca. _e._ _B. m. musculus_, [F] ad., 45462, USNM, Colima, Colima. _f._ _B. m. nigrescens_, [M] ad., 76834, USNM, Comitan, Chiapas. _g._ _B. m. pallidus_, [F] ad., 4802, Texas A & M, Axochiapan, Morelos. _h._ _B. m. pullus_, [F] ad., 71608, KU, 8 mi. S Condega, Nicaragua. _i._ _B. t. allex_, [F] ad., 45453, USNM, Colima, Colima.] [Illustration: PLATE 2 Photographs of skulls (_a-g_) in dorsal view of _Baiomys_. x 2.