Diagnosis.—Size large for the species; ground color of dorsum Chaetura Drab; individual guard hairs of dorsum black to base, distal fourth of hairs of underfur in posterior half of dorsum tipped with grayish-brown, proximal three-fourths Dark Neutral Gray; in anterior region of dorsum, posterior to ears, distal third of hairs grayish-brown and proximal two-thirds Dark Neutral Gray to base; sides slightly paler than dorsum; ground color of belly Neutral Gray, individual hairs of belly and throat tipped with Pallid Neutral Gray, basally Deep Neutral Gray to Dark Neutral Gray; tips of individual hairs of face Ochraceous-Tawny; lateral vibrissae whitish, dorsal and ventral vibrissae black to base; forefeet and hind feet sooty above and below, thigh bearing [Pg 646] some white-tipped hairs; tail near Chaetura Drab above, Pale Neutral Gray below; anterior part of jugal projecting slightly ventrally and forming small protuberance at point of articulation with maxillary part of zygoma; jugal extending anteriorly nearly to lacrimal. In most cranial measurements averaging as large as B. t. analogous. Average and extreme measurements of the type and three additional paratypes, all adults, are: total length, 105.5 (101-109); length of tail, 39.8 (35-42); length of body, 65.8 (63-68); length of hind foot, 14.3 (14-15); length of ear from notch, 11 (11); occipitonasal length, 18.1 (18.1-18.8); zygomatic breadth, 9.6 (9.3-9.8); postpalatal length, 6.5 (6.0-6.7); least interorbital breadth, 3.4 (3.3-3.6); length of incisive foramina, 4.0 (3.8-4.2); length of rostrum, 6.3 (6.1-6.4); breadth of braincase, 8.8 (8.6-8.9); depth of cranium, 6.7 (6.5-6.8); alveolar length of maxillary tooth-row, 3.2 (3.1-3.3); for photograph of skull, see [Plate 2d], and [Plate 4e].
Comparisons.—From B. t. taylori, B. t. fuliginatus differs in: dorsum slightly darker than in darkest taylori; tail densely haired, bicolored rather than unicolored; belly sooty to grayish rather than grayish to whitish; forefeet and hind feet sooty to grayish rather than flesh-colored; incisive foramina less bowed laterally, more nearly straight; interparietal compressed anteroposteriorly, less diamond-shaped.
From B. t. paulus, B. t. fuliginatus differs in: dorsum dusky to blackish rather than fawn color; belly sooty to grayish rather than buffy to whitish-gray; forefeet and hind feet sooty to grayish rather than whitish; zygoma more nearly forming a right angle with rostrum or skull, less tapered anteriorly; anterior part of jugal possessing ventral projection; jugal extending nearly to lacrimal on posterior surface of maxillary part of zygoma.
From B. t. analogous, B. t. fuliginatus differs in: mid-dorsal region blacker, less brownish; tail distinctly bicolored rather than unicolored to faintly bicolored; incisive foramina not constricted medially; presphenoid broader (at narrowest point); jugal differs much the same as it does from paulus; nasals anteriorly truncate instead of rounded.
Remarks.—Dalquest (1953:155-157) and Booth (1957:15) assigned all of the pygmy mice that they examined from the state of San Luis Potosí to B. t. taylori. Examination of all of the material that was available to Dalquest, plus additional specimens at the University of Kansas Museum of Natural History, reveals that there are three subspecies in San Luis Potosí. B. t. taylori occurs in the eastern part of the State at lower altitudes; B. t. analogous occurs to the southeast at higher altitudes; B. t. fuliginatus occurs in the northeastern part of the State in the Sierra Madre Oriental.
Specimens obtained from Ebano, Pujal, and Tamuín, representative of B. t. taylori, are much paler on the belly and on the ventral surface of the forefeet and hind feet than are specimens from Ciudad del Maíz, representative of B. t. fuliginatus. The tail in B. t. taylori is nearly unicolored and less hairy than in the paratypical series of fuliginatus. Specimens from 4 km. NE Ciudad Valles are nearly intermediate in color of the belly, dorsum, forefeet and hind feet, and tail, between the palest mice from the coastal plain and the darker mice in the mountains of the northeastern part of the State (specimens from El Salto average paler, however, than the type and paratypes). These specimens seem to be intergrades between B. t. taylori to the east on the coastal plain and fuliginatus to the northwest in the mountains. It seems best to refer the mice from 4 km. N Ciudad Valles to B. t. taylori on the basis of the average of external and cranial characters. Specimens from 6 mi. SW San Gerónimo, Coahuila, also referred to B. t. taylori, resemble in color the mice from 4 km. N Ciudad Valles. When more specimens are obtained from the front range of the Sierra Madre Oriental, at lower altitudes, the manner in which these two subspecies intergrade with one another will be better understood. At present, populations from higher altitudes in the mountains seem to represent a dark subspecies; populations from the coastal plain represent a pale subspecies, and those from the lower slopes and high valleys seemingly are intergrades. B. t. fuliginatus occurs in a somewhat limited strip of chernozem soil (or suelos negros of Tamayo, 1949: Carta de Suelos). The populations occurring at lower altitudes on the coastal plain are on generally paler soils.
Specimens examined.—Total 39, all from the Republic of México, as follows: San Luis Potosí: El Salto, 24 Mus. Nat. Hist., Louisiana State Univ., 7 Amer. Mus. Nat. Hist.; type locality, 8 (including the type).
Marginal records.—See specimens examined.
Baiomys taylori paulus (J. A. Allen)
Peromyscus paulus, J. A. Allen, Bull. Amer. Mus. Nat. Hist., 19:598, November 12, 1903; Elliot, Field Columb. Mus. Publ., 105(6): 136, July 1, 1905.