Chapter II: Part 2

The general plan of structure of the auditory ossicles in _Baiomys_ resembles that in _Calomys_, _Akodon_, and _Thaptomys_. The ossicles of _Calomys_ and _Thaptomys_, in particular, closely resemble the auditory ossicles of _Baiomys musculus_. The short process of the incus is knoblike in _Calomys_ and _Thaptomys_, and the general conformation of malleus and stapes in those two genera is nearly identical to that in _B. musculus_. In _Akodon_, the anterior and posterior crus of the stapes is more rounded than in _B. musculus_, resembling that in _B. taylori_.

_Reithrodontomys_ differ from _Baiomys_ in having a more elongate orbicular apophysis on the body of the malleus, an elongated short limb on the incus, and a stapes having anterior and posterior crura bowed as in mice of the genus _Peromyscus_.

In _Ochrotomys_, the orbicular apophysis of the malleus resembles the orbicular apophysis of _B. musculus_, but the short process of the incus is longer, resembling the short process of _B. taylori_. In general conformation of the malleus, incus, and stapes, _Ochrotomys_ shows closer resemblance to _B. taylori_ than to _B. musculus_.

In _Holochilus_ the anterior crus and posterior crus of the stapes are similar to those in _B. musculus_, but in shape and size of malleus and incus, _Holochilus_ differs considerably from _B. musculus_ and _B. taylori_.

In _Zygodontomys_, size and shape of the ossicles differ greatly from those of _Baiomys_.

In the genus _Peromyscus_, only _Peromyscus floridanus_ (subgenus _Podomys_) possesses a knoblike short process on the incus similar to that in _B. musculus_; representatives of the other subgenera examined possess an elongated short limb on the incus. The conformation of the ossicles of both _Onychomys_ and _Oryzomys_ appears to be more nearly like that in _Peromyscus_ than that of _Baiomys_.

On the basis of shape and size of auditory ossicles, _Baiomys_ resembles South American hesperomines (_Calomys_ and _Thaptomys_) rather than North American hesperomines.

Genus =Baiomys= True

1894. _Baiomys_ True, Proc. U. S. Nat. Mus., 16:758, February 7.
Type, _Hesperomys (Vesperimus) taylori_ Thomas.

_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.

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.

_Characters of the species._--Size large (extremes in external measurements of adults; total length, 100-135; length of tail vertebrae, 33-56; length of hind foot, 14.1-17; length of ear, 9-12); upper parts dark reddish brown, or ochraceous-buff to nearly black; underparts pale pinkish buff to white or pale buffy.

_Geographic variation._--Eight subspecies are here recognized (see Figure 10). Features that vary geographically are external size, color of pelage, certain cranial dimensions (occipitonasal length, zygomatic breadth, least interorbital breadth, length of rostrum, length of incisive foramina, depth and breadth of cranium, and alveolar length of upper molar tooth-row).

External and cranial size (except for _B. m. handleyi_) is less in the southernmost subspecies, _B. m. pullus_, _B. m. grisescens_, _B. m. nigrescens_, and more in the northernmost subspecies, _B. m. musculus_, _B. m. brunneus_, and _B. m. infernatis_. Increase in size from south to north is in keeping with Bergman's Rule that within a species, smaller individuals occur in warmer parts of its geographic range. Southern pygmy mice at high altitudes average larger than those from low elevations, except where the two species are sympatric. There the Southern Pygmy Mouse is uniformly larger, regardless of altitude.

Osgood (1909:257, 259) suggested that degree of relative humidity might in some way control color of pelage in both _B. taylori_ and _B. musculus_. In _B. musculus_, the darker subspecies, _B. m. brunneus_, _B. m. nigrescens_, and _B. m. pullus_, occur in zones of rather constant high relative humidity, whereas the paler subspecies _infernatis_, _musculus_, _handleyi_, and to a less extent _grisescens_ and _pallidus_, occur in zones of lower relative humidity. This is in keeping with Gloger's Rule, which states that melanins increase in the warm and humid parts of the range of a species, and reddish or yellowish-brown phaeomelanins prevail in arid climates. _B. m. musculus_ ranges into areas where relative humidity is such that darker pelages might be expected, but this is in the area where the two species are sympatric, and color of pelage may be an important character of recognition.

1. _B. m. brunneus_
2. _B. m. grisescens_
3. _B. m. handleyi_
4. _B. m. infernatis_
5. _B. m. musculus_
6. _B. m. nigrescens_
7. _B. m. pallidus_
8. _B. m. pullus_]

_Natural History_

_Habitat and numbers._--In Veracruz, Dalquest obtained the southern pygmy mouse in stands of tall grass (_Spartina?_) in sandy loam soil bordering, and in, dense vegetation; Davis (1944:394) found the species living in dense stands of grasses and seemingly utilizing underground burrows. Near Chilpancingo, Guerrero, rocky situations seemed to be the preferred habitat. Davis (_loc. cit._) believed that the species has a wide tolerance to kinds of habitats. In Morelos, Davis and Russell (1954:75) found these mice to be abundant along rock fences separating cultivated fields, and in arid lowlands. In Colima, Hooper (1955b:13) obtained specimens from an open thorn forest in sparse grass and rocky hillside bounding a stream and in litter below shrubs on the floor of a nut-palm forest; in Michoacan, these mice were taken in cane grass, shrubs, and mesquite near an irrigation ditch. From Guatemala, Goodwin (1934:39, 40) records specimens from Sacapulas, a hot, dry, sandy area where cactus and sparse grasses are present, and from La Primavera, on the edges of pine-oak-alder forests. Felten (1958:137) has taken _musculus_ from bushy areas in El Salvadore. In 1955, I obtained the southern pygmy mouse 6 mi. SW Izucar de Matemores, Puebla, along a stream in heavy grass bordered by cypress, willow, fig, bamboo, and in rocky grazed area near sugar cane fields.

The southern pygmy mouse seems to be locally abundant in certain parts of its geographic range, and in other parts, scarce. For example, Dalquest (_in. litt._) recorded the pygmy mouse as common at a place 2 km. N Paraje Nuevo, 1700 feet, Veracruz, where, by means of 50 traps, he took 14 of these mice in one night. The species was scarcer, although the habitat seemed suitable, 3 km. N Presidio, 1500 feet, Veracruz, where he caught only two pygmy mice in several days of trapping. Six miles southwest of Izucar de Matemores, the pygmy mouse was the most common rodent. I have trapped for it in Oaxaca and Veracruz in habitats that seemed almost identical to those mentioned by Dalquest, and also that at Izucar de Matemores, Puebla, with almost no success. The reason for the seeming disparity in numbers at different localities having nearly the same kind of habitat is unknown to me and bears further investigation.

_Behavior._--Little is recorded concerning the behavior of this species. David and Russell (_op. cit._:76) found that of small mammals _B. musculus_ was the first to appear at night. I caught mice of this species by hand in the afternoon in Puebla. They seemed to be active from noon until dark. Albert Alcorn wrote in his field notes that specimens were taken near noon at a place 9 mi. NNW Esteli, Nicaragua. My impression is that _musculus_ is diurnal to crepuscular.

_Enemies and food._--Owl pellets (thought to be those of a barn owl, _Tyto alba_) from within the geographic range of _B. musculus_, from 6 mi. SW Izucar de Matemores, yielded mandibular tooth-rows belonging to _musculus_. Presumably, most of the carnivorous mammals and raptorial birds within the range of the southern pygmy mouse could be listed as enemies. Diurnal to crepuscular habits of this mouse may protect it from some of the nocturnal carnivorous mammals and raptorial birds.

Food of the southern pygmy mouse includes nuts, bark, grass seeds, and leaves. Dalquest (MS) writes that bits of banana proved to be useful bait in trapping these mice in Veracruz.

_Reproduction._--Notations concerning lactation and embryos on specimen labels of females suggest that the southern pygmy mouse breeds in all months. I have records of pregnant or lactating females in every month, save January, April, May, and June. The average of 26 counts of embryos or young per litter is 2.92 (1-4).

=Baiomys musculus brunneus= (J. A. Allen and Chapman)

_Peromyscus musculus brunneus_ J. A. Allen and Chapman, Bull. Amer.
Mus. Nat. Hist., 9:203, June 16, 1897; Elliott, Field Columb. Mus.
Publ., 105(4):136, July 1, 1905; Elliott, Field Columb. Mus. Publ.,
115(8):203, 1907; Osgood, N. Amer. Fauna, 28:259, April 17, 1909.

_Baiomys musculus brunneus_, Miller, Bull. U. S. Nat. Mus., 79:137,
December 31, 1912; Miller, Bull. U. S. Nat. Mus., 128:318, April 29,
1924; Ellerman, The Families and Genera of Living Rodents, 2:402,
March 21, 1941; Goldman, Smith. Miscl. Coll., 115:437, July 31,
1951; Goodwin, Bull. Amer. Mus. Nat. Hist., 102:318, August 31,
1953; Miller and Kellogg, Bull. U. S. Nat. Mus., 205:512, March 3,
1955; Booth, Walla Walla Publs., Dept. Biol. Sci., 20:15, July 10,
1957; Hall and Kelson, The Mammals of North America, 2:661, March
31, 1959 (part).

[_Peromyscus musculus_] _brunneus_, Elliott, Field Columb. Mus.
Publ., 95(4): 176, 1904.

_Peromyscus musculus_ [_musculus_], Osgood, N. Amer. Fauna, 28:258,
April 17, 1909 (part).

_Baiomys musculus musculus_, Davis, Jour. Mamm., 25:394, December
12, 1944 (part); Goldman, Smith Miscl. Coll., 115:437, July 31,
1951; Hooper, Jour. Mamm., 33:97, February 18, 1952 (part); Hall and
Kelson, The Mammals of North America, 2:661, March 31, 1959 (part).

_B._ [_aiomys_] _m._ [_usculus_] _brunneus_, Hooper, Jour. Mamm.,
33:96, February 18, 1952.

_Baiomys taylori_, Hooper, Jour. Mamm., 33:97, February 18, 1952
(part).

_Type._--Adult female, skin and skull; No. 12535/10845 American Museum of Natural History; Jalapa, Veracruz, Republic of Mexico; obtained on April 13, 1897, by F. M. Chapman, original number 1203.

_Range._--Central Veracruz, coastal plains and eastern slopes of the plateau of Central Mexico, see Figure 10. Zonal range: Upper Tropical Life-zone (Lowery and Dalquest, 1951:537), parts of the Veracruz and eastern Transverse Volcanic biotic provinces of Goldman and Moore (1945:349). Occurs from near sea level at Boca del Rio, Veracruz, up to 5500 feet 3 km. SE Orizaba.

_Diagnosis._--Size medium to large for the species; ground color of dorsum of paratypes near Olive Brown; darkest of specimens of this subspecies examined (from Potrero Viejo, Veracruz) between Prouts Brown and Mummy Brown; distal two-thirds of guard hairs of dorsum black, proximal third dark gray to sooty; hairs of dorsum black-tipped having subterminal band of Ochraceous-Tawny; sides paler (less of dark brown) than dorsum; venter Deep Olive Buff to clay color, individual hairs pale olive buff at tips, dark gray basally; region of throat and chin sooty gray; ventralmost vibrissae white to base, other vibrissae black to base; ears dark brown, sparsely haired; forefeet and hind feet flesh-colored in palest specimens, sooty in darkest; tail pale brown, slightly paler below than above; presphenoid only slightly constricted towards midline; average and extreme external and cranial measurements of 10 adults from Cerro Gordo, Veracruz, are as follows: total length, 118.9 (112-127); length of tail vertebrae, 45.1 (42-50); length of body, 74.0 (69-78); length of hind foot, 16.0 (16); length of ear from notch, 12.8 (12-13); occipitonasal length, 19.5 (19.0-20.0); zygomatic breadth, 10.3 (10.0-10.8); postpalatal length, 7.1 (6.7-7.5); least interorbital breadth, 3.9 (3.7-4.0); length of incisive foramina, 4.4 (4.1-4.6); length of rostrum, 6.9 (6.5-7.2); breadth of braincase, 9.5 (9.2-9.7); depth of cranium, 7.1 (7.1-7.4); alveolar length of maxillary tooth-row, 3.3 (3.2-3.3); for photographs of skull, see Plate 1_a_, and Plate 3_a_.

_Comparisons._--For comparisons with _B. m. nigrescens_, see account of that subspecies. From _B. m. pallidus_, _B. m. brunneus_ differs in: dorsal, lateral, and facial coloration deeper reddish brown, more melanins present; venter darker; buff gray rather than whitish buff to gray as in paratypical series; vibrissae black rather than brownish to white; tail sooty, less flesh-colored; forefeet and hind feet averaging slightly grayer; most external and cranial dimensions averaging slightly larger; nasals less attenuated; presphenoid less hour-glass shaped, sides more nearly straight.

From _B. m. infernatis_, _B. m. brunneus_ differs in: side of face and neck deep reddish-brown rather than yellowish-gray (the differences in dorsal colorations are greater between _brunneus_ and _infernatis_ than between _brunneus_ and _pallidus_); venter darker buff-gray; tail brownish rather than flesh-colored; forefeet and hind feet average slightly grayer; most external dimensions averaging slightly larger; cranial dimensions nearly the same except length of incisive foramina, which is smaller; presphenoid differs in much the same way as from pallidus.

_Remarks_.--Specimens from Chichicaxtle, Puente Nacional, 3 km. W Boca del Rio, 1 km. E. Mecayucan, and Rio Blanco (20 km. WNW Piedras Negras), are all paler than the paratypical series and other specimens from within the assigned range of _B. m. brunneus_. All these specimens from the coastal plain average considerably paler than those from the front range and slopes of the mountains. Specimens from Puente Nacional are intermediate in color between paler, grayish brown, specimens from the coastal plains and the darker, brown, specimens from the mountains. When Allen and Chapman (1897:203) described _brunneus_, they did so on the basis of the darker brown mice from the higher altitudes. The name, _brunneus_, _sensu stricto_, could be restricted to those mice from the higher altitudes of central Veracruz. However, when the mice of intermediate color from Puente Nacional are considered, it seems best to include the material from the coastal plain with _brunneus_. Crania from the higher altitudes are slightly larger than, but not significantly different from, crania of specimens from the coastal plains. Specimens examined from the coastal plains resemble the darker series of _B. m. pallidus_ to the west in central Mexico. But there is no evidence of gene flow between the paler coastal specimens and _B. m. pallidus_ to the west. In fact, these paler brown mice on the coastal plain grade in color into the darker brown mice from the mountains. The paler mice from the coast may be an incipient subspecies.

The type and paratypes seem to have faded somewhat since they were described by Allen and Chapman (_loc. cit._) and by Osgood (1909:259). However, the color of the paratypes and other specimens herein assigned is the feature most useful for distinguishing _brunneus_ from all other subspecies of _B. musculus_.

_Specimens examined._--Total 187 all from VERACRUZ, Republic of Mexico, and distributed as follows: type locality, 4400 ft., 16[1] (including the type), 6[2], 1[3]; _Cerro Gordo_, 1500 ft., 19; _Teocelo_ [= _Texolo_], 4500 ft., 1; _2 mi. NW Plan del Rio_, 1000 ft., 14[4]; _Plan del Rio_, 1000 ft., 2[5]; _Carrizal_, 4[2]; Chichicaxtle, 3[2]; _Puente Nacional_, 500 ft., 1[5], 2; _Santa Maria, near Mirador_, 1800 ft., 10[2]; Boca del Rio, 10 ft., 1[5], 8; _Cordoba_ [= _Cordova_], 14[1]; _4 km. WNW Fortin_, 4; _Rio Atoyac, 8 km. NW Potrero_, 1; _2 km. N. Paraje Nuevo_, 1700 ft., 9; _El Xuchil_, _1 mi. W. Paraje Nuevo_, 6[6]; Potrero Viejo, 1700 ft. 15; _Cautlapan_ [= _Ixtaczequitlan_], 4000 ft., 16; _Micayucan_, 1; 3 km. SE Orizaba, 5500 ft., 3; Rio Blanco, 20 km. WNW Piedras Negras, 400 ft, 7; _29 km. SE Cordoba, Presidio_, 15[4]; _3 km. N Presidio_, 1500 ft., 2; Presidio, 600 meters, 6[3].

_Marginal records._--VERACRUZ: type locality; Chichicaxtle; Boca del Rio, 10 ft.; Rio Blanco, 20 km. WNW Piedras Negras, 400 ft; Presidio; 3 km. SE Orizaba, 5500 ft.

[1] American Museum of Natural History.

[2] U. S. Nat. Museum (Biol. Surv. Coll.).

[3] Chicago Natural History Museum.

[4] Univ. Michigan, Museum of Zoology.

[5] Texas A & M, Coop. Wildlife Res. Coll.

[6] Univ. Illinois, Mus. Nat. History.

=Baiomys musculus grisescens= Goldman

_Baiomys musculus griesescens_ Goldman, Proc. Biol. Soc. Washington,
45:121, July 30, 1932; Ellerman, The Families and Genera of Living
Rodents, 2:402, March 21, 1941; Poole and Schantz, Bull. U. S. Nat.
Mus., 178:259, March 6, 1942; Goodwin, Bull. Amer. Mus. Nat. Hist.,
79(2):160-161, May 29, 1942 (part); Miller and Kellogg, Bull. U. S.
Nat. Mus., 205:513, March 3, 1955 (part); Felten, Senck. Biol.,
39:136, August 30, 1958; Packard, Univ. Kansas Publs., Mus. Nat.
Hist., 9:401, December 19, 1958; Hall and Kelson, The Mammals of
North America, 2:661, March 31, 1959 (part).

_Type._--Adult female, skin and skull; No. 257083 U. S. Nat. Mus. (Biol. Surv. Coll.); Comayabuela [= Comayaguela] just south of Tegucigalpa, 3100 feet, Honduras; obtained on March 6, 1932, by C. F. Underwood, original number 838.

_Range._--Central to south-central Guatemala, east to south-central Honduras. Zonal range: Lower parts of the Merendon Biotic Province of Smith (1949:235). Occurs from 3200 feet at a place 1/2 mi. N and 1 mi. W Salama, Guatemala, up to approximately 4500 feet at Monte Redondo, Guatemala.

_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 Jesus 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 1_b_, and Plate 3_b_.

_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 Nenton and Jacaltenango, on the other hand, are darker brownish-black, more nearly like the paratypical series of _nigrescens_ from the Valley of Comitan, Chiapas, Republic of Mexico. 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 Rio 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 Jesus 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 Jesus 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 Jesus 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.

[9] Univ. Michigan, Museum of Zoology.

=Baiomys musculus handleyi= Packard

_Baiomys musculus handleyi_ Packard, Univ. Kansas Publs., Mus. Nat.
Hist., 9:399, December 19, 1958.

_Baiomys musculus musculus_, Goodwin, Bull. Amer. Mus. Nat. Hist.,
68(1):39-40, December 12, 1934 (part); Miller and Kellogg, Bull.
U. S. Nat. Mus., 205:512, March 3, 1955 (part).

_Baiomys musculus nigrescens_, Hall and Kelson, The Mammals of North
America, 2:661, March 31, 1959 (part).

_Type._--Adult female, skin and skull; No. 275604 U. S. Nat. Mus. (Biol. Surv. Coll.); Sacapulas, El Quiche, Guatemala; obtained on April 24, 1947, by Charles O. Handley, Jr., original number 991.

_Range._--Known only from the type locality in the valley of the Rio Negro. Zonal range: Part of the Chimaltenangan Province of Smith (1949:235).

_Diagnosis._--Size medium to large for the species; dorsum Wood Brown in some series to Buffy Brown; guard hairs of dorsum black-tipped, color of underhairs Avellaneous; hairs white to base in region of chin, throat, and median venter; in lateral region, hairs Neutral Gray at base; dorsal surfaces of forefeet and hind feet and ankles white; tail white below, brownish above; nasals truncate anteriorly; frontoparietal suture forming an obtuse angle with the suture separating the parietals; alveolar length of upper molar tooth-row and tail long. Average and extreme external and cranial measurements for nine adults from the type locality are as follows: Total length, 121.4 (115-128); length of tail vertebrae, 50.7 (49-54); length of body, 70.8 (66-77); length of hind foot, 15.3 (15-16); occipitonasal length, 19.6 (18.8-20.7); zygomatic breadth, 10.5 (10.2-11.0); postpalatal length, 6.9 (6.4-7.4); least interorbital breadth, 4.0 (3.9-4.0); length of incisive foramina, 4.2 (4.0-4.5); length of rostrum, 7.2 (7.0-7.7); breadth of braincase, 9.8 (9.7-10.2); depth of cranium, 7.1 (6.8-7.2); alveolar length of maxillary tooth-row, 3.5 (3.4-3.6); for photographs of skull, see Plate 1_c_, and Plate 3_c_.

_Comparisons._--From _B. m. nigrescens_, _B. m. handleyi_ differs as follows: everywhere paler; forefeet and hind feet whitish instead of dusky to sooty; hairs of anterior part of face white instead of brown; tail bicolored instead of unicolored; anterior tips of nasals truncate rather than rounded; frontoparietal suture forming obtuse angle with suture separating parietals instead of forming right angle; tail and upper molar tooth-row longer.

From _B. m. grisescens_, _B. m. handleyi_ differs in: slightly paler above and below, primarily as a result of lacking buff-colored hairs; forefeet and hind feet white, not flesh-colored with gray overtones; tail bicolored, not unicolored; anterior tips of nasals truncate rather than flaring; tail and upper molar tooth-row longer.

_Remarks._--_B. m. handleyi_ seems to be restricted to the valley of the Rio Negro, in the region of Sacapulas, Guatemala. Stuart (1954:7) points out that the Rio Negro drops down into a gorge at a place near Sacapulas and flows northward through a deep canyon for approximately 60 kilometers. The Rio Negro, then, flows onto the lowlands of the Yucatan Peninsula. The habitat is xerophytic in the valley of the Rio Negro near Sacapulas. Stuart (_op. cit._:10) suggests that this xerophytic habitat may be continuous to a place to the north of Chixoy, Chiapas, where the vegetation then becomes more mesic. The mesic conditions to the north in Tabasco and Yucatan probably have restricted the movement of pygmy mice to the north. No specimens of this mouse are known from the Yucatan Peninsula or from the State of Tabasco, Mexico. _B. m. handleyi_ intergrades with _B. m. grisescens_ to the south. Specimens from 1 mi. S Rabinal, and those from a second locality 1/2 mi. N and 1 mi. E Salama, Guatemala, are intermediate in color of pelage between _handleyi_ and _grisescens_. Stuart (_op. cit._:5) mentions the continuity of habitat and tributaries from the Salama Basin into the valley of the Rio Negro. Absence of physiographic and biotic barriers in the corridor between these two basins probably allows for some gene flow between _handleyi_ and _grisescens_, and results in populations intermediate in color. To the north and northwest of Sacapulas, the Sierra de los Cuchumatanes rises abruptly and separates the known geographic range of _handleyi_ from that of _nigrescens_ to the north, while to the west the cactus-mesquite habitat of _handleyi_ gives way to the oak-pine timber that, so far as known, does not support _Baiomys_. The difference in elevation and flora seems to restrict gene flow between _handleyi_ and the more northern _nigrescens_. The only evidence of integration between these two subspecies is provided by one specimen from Chanquejelve, Guatemala. That specimen is intermediate in color between the pale _handleyi_ and blackish-brown _nigrescens_.

The subspecies closest, geographically, to _B. m. handleyi_ is _B. m. nigrescens_, from which _B. m. handleyi_ differs more in color than from any of the other named subspecies, except _B. m. pullus_. There is a close correlation of pallor of mice and the xeric Rio Negro Valley, and the darkness (melanistic color) of mice and the mesic mountains and valleys to the north.

_Specimens examined._--Total 49, from GUATEMALA: type locality, including the type: 12 (U. S. Nat. Mus., Biol. Surv. Coll.), 37 (Amer. Mus. Nat. Hist.).

=Baiomys musculus infernatis= Hooper

_Baiomys musculus infernatis_ Hooper, Jour. Mamm., 33:96, February
18, 1952; Miller and Kellogg, Bull. U. S. Nat. Mus., 205:512,
March 3, 1955; Hall and Kelson, The Mammals of North America,
2:661, March 31, 1959.

_Baiomys musculus musculus_, Hooper, Jour. Mamm., 28:50, February
15, 1947 (part).

_Type._--Adult male, skin and skull; No. 91497 Univ. of Michigan, Museum of Zoology; Teotitlan, Oaxaca, Republic of Mexico, obtained on February 24, 1947, by Helmuth O. Wagner, original number 2702.

_Range._--Southeastern Puebla, in the basin drained by the Rio Salado and Rio Quiotepec, into northern Oaxaca. Zonal range: Arid Tropical in a part of the Orizaba-Zempoaltepec Faunal District of the Transverse Volcanic Biotic Province of Moore (1945:218). Occurs from 3100 feet in Oaxaca up to 6000 feet in Puebla.

_Diagnosis._--Size medium for the species; dorsum Drab, terminal parts of individual guard hairs black, Neutral Gray basally, distal parts of underfur Pinkish Buff, proximally Neutral Gray; sides same color as dorsum; hairs in region of throat and chin white to base; venter whitish to Neutral Gray with tinges of Pinkish Buff; dorsal parts of forefeet and hind feet whitish with flesh-colored undertones, ventral parts whitish to dusky-gray; tail bicolored, grayish-brown above, white below; tip of tail not bicolored, instead grayish-brown throughout; ears pale brown, sparsely haired; incisive foramina long, not constricted posteriorly. Average and extreme external measurements for 9 adults from the type locality are as follows: total length, 113.9 (106-122); length of tail vertebrae, 44.1 (41-48); length of body, 71.0 (65-79); length of hind foot, 14.8 (13-16); length of ear, 11.9 (11-12). Average and extreme cranial measurements of 7 adults from the type locality are as follows: Occipitonasal length, 20.1 (19.7-20.4); zygomatic breadth, 10.4 (10.2-10.6); postpalatal length, 7.3 (7.0-7.7); least interorbital breadth, 4.2 (4.0-4.4); length of incisive foramina, 4.8 (4.4-5.6); length of rostrum, 7.2 (6.6-7.5); breadth of braincase, 9.6 (9.5-9.8); depth of cranium, 7.4 (7.1-7.6); alveolar length of maxillary tooth-row, 3.3 (3.1-3.4); for photographs of skull, see Plate 1_d_, and Plate 3_d_.

_Comparisons._--For comparisons with _B. m. nigrescens_ and _B. m. brunneus_, see accounts of those subspecies. From _B. m. pallidus_, _B. m. infernatis_ differs in: sides, ears, and dorsum paler (less of dark brown); venter whitish gray rather than gray with tinge of buff and brown; forefeet and hind feet paler; tail bicolored, not unicolored; incisive foramina longer and not constricted posteriorly; mastoid process turning dorsally and sickle-shaped at posteriormost point rather than capitate.

_Remarks._--_B. m. infernatis_ resembles _B. m. handleyi_ more than any other subspecies in color of pelage and in external and cranial dimensions. The resemblance in color between _B. m. pallidus_, in certain parts of its range, and _B. m. handleyi_ may have resulted from nearly parallel selective forces that gave rise to two subspecies, widely separated geographically. The same relation obtains between _B. m. infernatis_ and _B. m. handleyi_. Both inhabit arid river basins. In them, pale soil and low relative humidity are important passive factors of selection that give adaptive value to the pale colors of pelage of both _infernatis_ and _handleyi_.

Specimens from 6-1/2 mi. SW Izucar de Matemores, and 1 mi. SSW Tilapa, Puebla, are intergrades between _B. m. infernatis_ and _B. m. pallidus_. These specimens are intermediate in color and cranial characters between the aforementioned subspecies but possess more of the pale brown overtones seen in paratypes of _pallidus_, and are best referred to that subspecies.

_Specimens examined_ (All in Univ. Michigan, Mus. Zool.).--Total 18, all from the Republic of Mexico and distributed as follows: PUEBLA, Tepanaco, 6000 ft., 3, Tehuacan, 5400 ft., 3. OAXACA: Type locality, 3100 ft., 12 (including the type).

_Marginal records._--See specimens examined.

=Baiomys musculus musculus= (Merriam)

_Sitomys musculus_ Merriam, Proc. Biol. Soc. Washington, 7:170,
September 29, 1892; Lyon and Osgood, Bull. U. S. Nat. Mus.,
62:135, January 15, 1909.

_Baiomys musculus_ [= _musculus_], Mearns, Bull. U. S. Nat. Mus.,
56:381, April 13, 1907; Hooper, Jour. Mamm., 36:29, May 26, 1955.

_Peromyscus musculus_ [_musculus_], J. A. Allen and Chapman, Bull.
Amer. Mus. Nat. Hist., 9:203, June 16, 1897; Elliot, Field Columb.
Mus. Publ., 105(4):135, July 1, 1905; Osgood, N. Amer. Fauna,
28:257, April 17, 1909 (part).

[_Peromyscus_] _musculus_, Trouessart, Cat. Mamm., 1:518, 1898.

[_Peromyscus_] _musculus_ [_musculus_], Elliot, Field Columb. Mus.
Publ., 95(4):175, July 15, 1904.

_Baiomys musculus musculus_, Miller, Bull. U. S. Nat. Mus., 79:137,
December 31, 1912 (part); Miller, Bull. U. S. Nat. Mus., 128:318,
April 29, 1924 (part); Ellerman, The Families and Genera of Living
Rodents, 2:402, March 21, 1941; Poole and Schantz, Bull. U. S. Nat.
Mus., 178:258, March 6, 1942; Davis, Jour. Mamm., 25:394, December
12, 1944 (part); Hooper, Jour. Mamm., 28:50, February 15, 1947
(part); Hall and Villa-R., Univ. Kansas Publs., Mus. Nat. Hist.,
1:460, December 27, 1949 (part); Hall and Villa-R., Anal. del Inst.
Biol., 21:196, September 28, 1950 (part); Goldman, Smith. Miscl.
Coll., 115:336, July 31, 1951 (part); Miller and Kellogg, Bull.
U. S. Nat. Mus., 205:512, March 3, 1955 (part); Hooper, Occas.
Papers Mus. Zool. Univ. Michigan, 565:13, March 31, 1955; Hall and
Kelson, The Mammals of North America, 2:661, March 31, 1959 (part).

_B._ [_aiomys_] _m._ [_usculus_] _musculus_, Hooper, Jour. Mamm.,
33:97, February 18, 1952 (part); Packard, Univ. Kansas Publs.,
Mus. Nat. Hist., 9:400; December 19, 1958.

_Baiomys taylori allex_, Hall and Kelson, The Mammals of North
America, 2:659, March 31, 1959 (part).

_Type._--Adult female, skin and skull; No. 33437/45460 U. S. Nat. Mus. (Biol. Surv. Coll.); Colima (City), Colima, Republic of Mexico, obtained on March 9, 1892, by E. W. Nelson, original number 2055.

_Range._--Southwestern Nayarit and northwestern Jalisco, south into Colima, thence eastward into Michoacan. Zonal range: part of arid Lower Tropical Subzone of Goldman (1951:330); approximates part of the Nayarit-Guerrero Biotic Province of Goldman and Moore (1945:349). Occurs from near sea level in Colima up to 5800 feet in Jalisco.

_Diagnosis._--Size large for the species; dorsum Olive-Brown in darkest series to Buffy Brown with tones of Fawn Color in the palest series; guard hairs of dorsum black-tipped, gray basally (in some specimens, guard hairs gray-tipped with subterminal black band, and gray base); underfur of dorsum black-tipped with subterminal band of fawn to buff, Neutral Gray basally; face and head paler than back because of greater number of fawn-colored and buff-colored hairs; hairs on throat and chin white to base; venter and flanks Pale Olive-Buff in palest series to Gray (Pale Gull Gray) in darkest series; individual hairs of venter tipped with white to buff, basally Gray (Dark Gull Gray); forefeet and hind feet white to gray with flesh-colored undertones; tail faintly bicolored, individual hairs above black, below white; nasals flared anteriorly; zygoma and zygomatic plate thick. Average and extreme external and cranial measurements for 8 adults from Armeria, Colima, are as follows: total length, 125.5 (115-135); length of tail vertebrae, 47.5 (42-54); length of body, 75.6 (68-81); length of hind foot, 16.5 (16-17); occipitonasal length, 20.3 (19.8-20.7); zygomatic breadth, 10.7 (10.3-11.1); postpalatal length, 7.4 (7.1-7.7); least interorbital breadth, 4.0 (3.9-4.1); length of incisive foramina, 4.3 (4.1-4.5); length of rostrum, 7.3 (6.9-7.6); breadth of braincase, 9.8 (9.4-10.0); depth of cranium, 7.1 (6.7-7.2); alveolar length of maxillary tooth-row, 3.4 (3.3-3.6); for photographs of skull, see Plate 1_e_, and Plate 3_e_.

_Comparisons._--For comparisons with _B. m. brunneus_, _B. m. infernatis_, and _B. m. pallidus_, see accounts of those subspecies. From _B. m. nigrescens_, _B. m. musculus_ differs in: dorsum paler throughout (less of blackish brown); region of face and ears paler, more buff and fawn-colored hairs rather than blackish-brown to grayish hairs; vibrissae paler; venter paler, less dark gray and less of sooty-colored undertones, tips of hairs whitish to pale Olive-Buff rather than light gray at tips becoming darker basally; forefeet and hind feet paler, whitish to pale buff-color with flesh-colored undertones, not sooty-colored to dark brown; tail paler below; nasals flaring outward, not tapering toward midline at anteriormost point; zygoma more massive; larger in external and cranial dimensions.

_Remarks._--Merriam (1892:170) described _Sitomys_ [= _Baiomys_] _musculus_ on the basis of 23 specimens (from Colima City, Colima; Armeria, Colima; Plantinar, and Zapotlan, Jalisco). According to the original description, _B. musculus_ resembled a small house mouse and was smaller than any known species of _Sitomys_ except _S. taylori_ [= _Baiomys taylori_]. From _taylori_, _musculus_ differed in being larger [in size of body], and in having longer ears and tail, and larger hind feet. When Allen and Chapman (1897:203) described _Peromyscus_ [= _Baiomys_] _musculus brunneus_ from Jalapa, Veracruz, the specimens described by Merriam from Colima and Jalisco became representative of the nominal subspecies _B. m. musculus_. Osgood (1909:258) assigned specimens from Colima, Guerrero, Jalisco, Michoacan, Morelos, Oaxaca, Puebla, Sinaloa, Veracruz, and Zacatecas to the subspecies _musculus_. Subsequently, Russell (1952:21) named the subspecies _pallidus_ from the arid lowlands of Morelos; Hooper (1952:96) described the subspecies _infernatis_ from northern Oaxaca and southeastern Puebla; and Goodwin (1959:1) described a new subspecies _nebulosus_ from the Oaxaca highlands. Each of the subspecies mentioned immediately above was described from within the geographic range assigned to _B. m. musculus_ by Osgood (_loc. cit._). Hall and Kelson (1959:661) mapped the range of _B. m. musculus_ so as to include Colima, parts of Jalisco, Michoacan, Guerrero, Oaxaca, and Veracruz. Lukens (1955:159), in a study of the mammals of Guerrero, has shown that the characters attributed to _B. m. pallidus_ are not significantly different from those of pygmy mice studied from Guerrero. He (_loc. cit._) concluded that: (1) if the specimens of pygmy mice from central Guerrero were typical of the subspecies _musculus_, then _pallidus_ did not deserve subspecific recognition, or; (2) the name _B. m. musculus_ should be restricted to the larger pygmy mice inhabiting the lowlands immediately adjacent to the Pacific Coast and the area to the north. My data (see Figure 12) show pygmy mice from southwestern Nayarit, northwestern and central Jalisco, Colima, and parts of Michoacan to be significantly larger in certain cranial and external measurements than pygmy mice from Guerrero, Oaxaca, Morelos, and parts of Puebla. This finding essentially corroborates Hooper's (1952a:96) findings. It seems advisable, therefore, to restrict the range of _B. musculus musculus_ to the large mice inhabiting west-central Mexico and the coastal lowlands of Colima and Michoacan. The name _pallidus_ is applicable to the smaller mice occupying Morelos, southwestern Puebla, Guerrero, Oaxaca, and southwestern Chiapas.

_B. m. musculus_ intergrades with _B. m. pallidus_ in eastern Michoacan and central and western Guerrero. Specimens from San Jose Prura and 12 mi. S Tzitzio, Michoacan, though referable to _B. m. musculus_ because of slightly larger size of crania are intermediate in size and color between the smaller and slightly darker _pallidus_ to the south and east and the larger, slightly paler _musculus_ to the northwest.

_Specimens examined._--Total 156 all from the Republic of Mexico, and distributed as follows: NAYARIT: 3 mi. NNW Las Varas, 150 ft., 1. JALISCO: 7 mi. W Ameca, 4000 ft., 2[10]; _6 mi. W Ameca_, 4300 ft., 3[10]; _10 mi. S Ameca_, 5800 ft., 1[10]; _13 mi. S, 15 mi. W Guadalajara_, 3; _13 mi. S, 9-1/2 mi. W Guadalajara_, 1; _3 mi. ENE Santa Cruz de las Flores_, 1; 27 mi. S, 12 mi. W Guadalajara, 1; _4 mi. NE Autlan_, 3000 ft., 5[10]; _Sierra de Autlan_, 5000 ft., 2[10]; _2-1/2 mi. NNE Autlan_, 3000 ft., 8; 2 mi. SSE Autlan, 1; _5 mi. S Purificacion_, 2; Chamela Bay, 1[10]; _2 mi. N La Resolana_, 1500 ft., 6[10]; _1 mi. N San Gabriel_, 4000 ft., 32[10]; 2 mi. N Cuidad Guzman, 5000 ft., 1; 3 mi. E Navidad, 4300 ft., 10[10]. COLIMA: _type locality_, 10[11] (including the type); _3 mi. SE Colima_ (_City_), 5[10]; _4 mi. SW Colima City_, 1; Armeria, 200 ft., 8[11]; _Paso del Rio_, 20[10].

MICHOACAN: 12 mi. S Tzitzio, 6[10]; San Jose Prura, 4[12]; 1 mi. E, 6 mi. S Tacambaro, 4000 ft., 3[13]; La Salada, 3[11]; 1/2 mi. SE Coalcoman, 15[10].

_Marginal records._--NAYARIT: 3 mi. NNW Las Varas, 150 ft. JALISCO: 3 mi. E Navidad, 4300 ft.; 27 mi. S, 12 mi. W Guadalajara. MICHOACAN: 12 mi. S Tzitzio; San Jose Prura; 1/2 mi. SE Coalcoman. COLIMA: Armeria, 200 ft. JALISCO: Chamela Bay.

[10] Univ. Michigan, Museum of Zoology.

[11] U. S. Nat. Museum (Biol. Surv. Coll.).

[12] Chicago Natural History Museum.

[13] Univ. California, Mus. Vert. Zoology.

=Baiomys musculus nigrescens= (Osgood)

_Peromyscus musculus nigrescens_ Osgood, Proc. Biol. Soc.
Washington, 17:76, March 21, 1904; Elliot, Field Columb. Mus. Publ.,
105(4):136, July 1, 1905; Lyon and Osgood, Bull. U. S. Nat. Mus.,
62:135, January 15, 1909; Osgood, N. Amer. Fauna, 28:259, April 17,
1909.

_Baiomys musculus nigrescens_, Miller, Bull. U. S. Nat. Mus.,
79:137, March 31, 1912; Miller, Bull. U. S. Nat. Mus., 128:318,
April 29, 1924; Goodwin, Bull. Amer. Mus. Nat. Hist., 68(1):40,
December 12, 1934; Ellerman, The Families and Genera of Living
Rodents, 2:402, March 21, 1941; Poole and Schantz, Bull. U. S. Nat.
Mus., 178:259, March 6, 1942; Hooper, Jour. Mamm., 28:50, February
15, 1947; Goldman, Smith. Miscl. Coll., 115:357, July 31, 1951;
Miller and Kellogg, Bull. U. S. Nat. Mus., 205:513, March 3, 1955;
Booth, Walla Walla Publs., Dept. Biol. Sci., 20:15, July 10, 1957;
Hall and Kelson, The Mammals of North America, 2:661, March 31, 1959
(part).

[_Peromyscus musculus_] _nigrescens_, Elliot, Field Columb. Mus.
Publ., 95(4):176, 1904.

_B._ [_aiomys_] _m._ [_usculus_] _nigrescens_, Goodwin, Bull. Amer.
Mus. Nat. Hist., 79(2):160, May 29, 1942; Hooper, Jour. Mamm.,
33:97, February 18, 1952 (part); Packard, Univ. Kansas Publs.,
Mus. Nat. Hist., 9:399, December 19, 1958.

_B._ [_aiomys_] _m._ [_usculus_] _musculus_, Booth, Walla Walla
Publs., Dept. Biol. Sci., 20:15, July 10, 1957 (part).

_Type._--Adult female, skin and skull; No. 76827 U. S. Nat. Mus. (Biol. Surv. Coll.); Valley of Comitan, Chiapas, Republic of Mexico, obtained on December 9, 1895, by E. W. Nelson and E. A. Goldman, original number 8719.

_Range._--Southern coastal region and eastern parts of Chiapas, southeastward into central and southern Guatemala, thence south into El Salvador (see Figure 10). Zonal range: parts of Lower Austral; also occurs in parts of the arid division of the Upper Tropical Life-zone, and in parts of the arid division of the Lower Tropical Life-zone; approximates a part of the Chiapas Highlands Biotic Province of Goldman and Moore (1945:349), and parts of the Guatemalan Subregion of Smith (1949:235).

_Diagnosis._--Size medium to small for the species; dorsum Vandyke Brown mixed with blackish, individual hairs black-tipped with a subterminal band of Warm Buff, Neutral Gray at base; guard hairs of dorsum black distally, Neutral Gray basally; hairs on sides grayish-brown, facial region like dorsum; chin buffy-brown; vibrissae brown, ventrally some white; venter creamy-buff to grayish, individual hairs creamy-buff at tips, gray basally; in region of throat and chin, hairs tipped with Ochraceous-Buff; dorsal surface of forefeet and hind feet dull whitish gray to brownish-black; tail indistinctly bicolored, dusky above, grayish to brownish below; incisive foramina short, wide medially; average and extreme external and cranial measurements of 15 adults from 6 mi. NW Tonala, Chiapas, are as follows: total length, 107.5 (100-116); length of tail vertebrae, 41.1 (33-48); length of body, 66.1 (62-73); length of hind foot, 15.0 (14-16); length of ear, 10.9 (10-12); occipitonasal length, 18.9 (18.4-19.7); zygomatic breadth, 9.8 (9.4-10.2); postpalatal length, 6.9 (6.6-7.4); least interorbital breadth, 3.7 (3.5-3.8); length of incisive foramina, 4.4 (4.1-4.8); length of rostrum, 6.7 (6.1-7.1); breadth of braincase, 9.2 (9.0-9.4); depth of cranium, 6.9 (6.5-7.3); alveolar length of maxillary tooth-row, 3.1 (2.9-3.2); for photographs of skull, see Plate 1_f_, and Plate 3_f_.

_Comparisons._--For comparisons with _B. m. handleyi_, _B. m. grisescens_, _B. m. musculus_, _B. m. pallidus_, and _B. m. pullus_, see accounts of those subspecies.

From _B. m. brunneus_, _B. m. nigrescens_ differs in: dorsum blackish-brown rather than reddish to ochraceous brown; face and ears brownish-black rather than brownish with tinges of ochraceous; vibrissae darker; forefeet and hind feet darker; venter with more grayish tones; dorsalmost part of zygomatic plate projects farther anteriorly; interparietal oval to diamond-shaped and narrower anteroposteriorly; zygomata narrower at anteriormost part; slightly smaller in most cranial and external measurements.

From _B. m. infernatis_, _B. m. nigrescens_ differs in: dorsum darker; region of face and ears darker; venter buffy to gray rather than whitish-buff; vibrissae darker; forefeet and hind feet darker; tail darker above and below; incisive foramina shorter, more constricted laterally; cranium slightly smaller in most dimensions.

_Remarks._--Hooper (1952a:93-94) reported specimens from the coastal strip of southern Chiapas as the most intensely pigmented, whereas, specimens from central and western Chiapas were distinctly paler. Crania of specimens from the coastal region of southern Chiapas were smaller than crania from the central highlands and mountains of Chiapas. My studies essentially corroborate the findings of Hooper. The gradation of color between the pale brown _pallidus_ to the north in Oaxaca, and the brownish-black _nigrescens_ to the south in Chiapas is extremely gradual. Specimens from the central and western parts of Chiapas (see Figure 10 for localities) are difficult to assign to either _pallidus_ or _nigrescens_. Equal justification exists for assignment to either subspecies. I have assigned the specimens to _nigrescens_ because they are geographically closer to the type locality of _nigrescens_. Specimens from Reforma, Oaxaca (assigned by Hooper, 1952a:93-94, to _nigrescens_), are nearly identical in size and color to paratypes of _pallidus_. I assign the Reforma specimens to _pallidus_.

The darkest of all the specimens examined and assigned to _nigrescens_ are from 1 mi. NW San Salvador and 1 mi. S Los Planes, El Salvador. The variations in color in this subspecies closely correspond to degree of relative humidity; the palest samples are from areas of low relative humidity and the darkest are from areas of high relative humidity. In view of the present state of differentiation of specimens from the southern coastal areas of Chiapas and mountainous areas of El Salvador, it would seem that populations there might be incipient subspecies.

_Specimens examined._--Total 319. CHIAPAS: _17 mi. W Bochil_, 1[14]; _15 mi. W Bochil_, 1[14]; _14 mi. W Bochil_, 1[14]; Bochil, 6[15]; Ocuilapa, 3500 ft., 5[16]; _5 mi. NNW Tuxtla Gutierrez_, 9; _11 km. W Tuxtla Gutierrez_, 800 m., 2[15]; _10 km. W Tuxtla Gutierrez_, 800 m., 2[15]; _Tuxtla Gutierrez_, 2600 ft., 8[16], 11; _Ocozocoautla_, 10[15], 2[16]; 25 mi. E Comitan, Las Margaritas, 1250 m., 5[17], 24[15]; Cintalpa, 555 m., 1[14], 18[15], 3[17]; _Jiquilpilas_, 2000 ft., 1[16]; San Bartolome, 3[16]; _type locality_, 5700 ft., 26[16] (including the type); 15 mi. SW Las Cruces, 1; Villa Flores, 600 m., 12[15]; _23 mi. S Comitan_, 1[14]; _15 mi. S, 2 mi. E La Trinitaria_, 4; _30 mi. S Comitan_, 2[14]; 35 mi. S Comitan, 1[14]; _3 mi. E Arriga_, 1[14]; 6 mi. NW Tonala, 19; _Tonala_, 8[16]; _Los Amates_, 1[14]; Pijijiapan, 10 m., 7[15]; Mapastepec, 45 m., 25[15], 4[17].

GUATEMALA: Chanquejelve, 1[14]; _Nenton_, 3000 ft., 1[16]; Jacaltenango, 5400 ft., 8[16]; La Primavera, 5[14]; 4 mi. S Guatemala City, 4700 ft., 3; _5 mi. S Guatemala City_, 4050 ft., 10; _6 mi. S Guatemala City_, 4680 ft., 1; _Lake Amatitlan_, 4500 ft., 13[16]; El Progresso (Distrito Santa Rosa), 3[15]; _2 mi. N, 1 mi. W Cuilapa_, 2980 ft., 1[14]; _1 mi. WSW El Molino_ (_Distrito Santa Rosa_), 2; _2-1/2 mi. W, 2-1/4 mi. N San Cristobal_, 2900 ft., 1; El Zapote, 1[15].

EL SALVADOR: 1 mi. NW San Salvador, 29; 1 mi. S Los Planes, 15.

_Marginal Records._--CHIAPAS: Bochil; 25 mi. E Comitan, Las Margaritas, 1250 ft. GUATEMALA: Chanquejelve; La Primavera; Jacaltenango, 5400 ft.; 4 mi. S Guatemala City, 4700 ft.; El Progresso. _El Salvador_: 1 mi. NW San Salvador; 1 mi. S Los Planes. GUATEMALA: El Zapote. CHIAPAS: Mapastepec, 45 m.; Pijijiapan, 10 m.; 6 mi. NW Tonala; 15 mi. SW Las Cruces; Cintalpa, 555 m.; Ocuilapa, 3500 ft.

[14] American Museum of Natural History.

[15] Univ. Michigan, Museum of Zoology.

[16] U. S. Nat. Museum (Biol. Surv. Coll.).

[17] University of Florida Collections.

=Baiomys musculus pallidus= Russell

_Baiomys musculus pallidus_ Russell, Proc. Biol. Soc. Washington,
January 29, 1952; Davis and Russell, Jour. Mamm., 35:75, February
10, 1954; Miller and Kellogg, Bull. U. S. Nat. Mus., 205:512; Hall
and Kelson, The Mammals of North America, 2:662, March 31, 1959.

_Peromyscus musculus brunneus_, Elliot, Field Columb. Mus. Publ.,
115(8):203, 1907 (part).

_Peromyscus musculus_ [_musculus_], Osgood, N. Amer. Fauna, 28:257,
April 17, 1909 (part).

_Baiomys musculus musculus_, Miller, Bull. U. S. Nat. Mus., 79:137,
December 31, 1912 (part); Miller, Bull. U. S. Nat. Mus., 128:318,
April 29, 1924 (part); Davis, Jour. Mamm., 25:394, December 12, 1944
(part); Hooper, Jour. Mamm., 28:50, February 15, 1947 (part);
Goldman, Smith, Miscl. Coll., 115:336, July 31, 1951 (part); Miller
and Kellogg, Bull. U. S. Nat. Mus., 205:512, March 3, 1955 (part);
Booth, Walla Walla Publs., Dept. Biol. Sci., 20:15, July 10, 1957
(part); Hall and Kelson, The Mammals of North America, 2:661,
March 31, 1959 (part); Goodwin, Amer. Mus. Novitates, 1929:1,
March 5, 1959.

_B._ [_aiomys_] _m._ [_usculus_] _musculus_, Hooper, Jour. Mamm.,
33:97, February 18, 1952 (part).

_B._ [_aiomys_] _m._ [_usculus_] _nigrescens_, Hooper, Jour. Mamm.,
33:97, February 18, 1952 (part).

_Baiomys musculus nebulosus Goodwin_, Amer. Mus. Novitates, 1929,
March 5, 1959.

_Type._--Adult female, skin and skull; No. 4501 Texas A&M Cooperative Wildlife Collection; 12 kms. NW Axochiapan, 3500 feet, Morelos, Republic of Mexico, obtained on July 28, 1950, by W. B. Davis, original number 5112.

_Range._--Guerrero thence eastward into Morelos and west central Puebla along the southern edge of the Transverse Volcanic Biotic Province (Goldman and Moore, 1945:349), south into Oaxaca, see Figure 10. Zonal range: largely Arid Lower Tropical Subzone of Goldman (1951:330). Occurs from near sea level in Oaxaca and Guerrero up to 6550 feet in Oaxaca.

_Diagnosis._--Size medium for the species; dorsum Buffy Brown in palest series to Olive-Brown in darkest series, individual hairs Warm Buff, Neutral Gray basally, some with black tips and a subterminal band of Warm Buff, guard hairs of dorsum black-tipped, gray basally; hairs on sides creamy-buff, gray basally; face same color as back fading to white on throat; vibrissae white-tipped, pale brown basally; venter, whitish with tinges of buff on lower throat, individual hairs having tips white to buffy-white, light gray basally; dorsal surface of forefeet and hind feet whitish to flesh-color; tail indistinctly bicolored, brownish above, grayish brown below; zygoma bowed as in _B. m. grisescens_; tail short; average and extreme external and cranial measurements for 17 adults from Tehuantepec, Oaxaca, are: total length, 117.3 (110-126); length of tail vertebrae, 46.9 (41-51); length of body, 70.4 (65-76); length of hind foot, 15.8 (15-16); occipitonasal length, 18.9 (18.2-20.1); zygomatic breadth, 10.1 (9.7-10.6); postpalatal length, 6.9 (6.6-7.5); least interorbital breadth, 3.8 (3.6-3.9); length of incisive foramina, 4.4 (4.2-4.7); length of rostrum, 6.7 (6.3-7.2); breadth of braincase, 9.3 (8.7-9.7); depth of cranium, 6.6 (6.4-6.8); alveolar length of maxillary tooth-row, 3.2 (3.1-3.4); for photographs of skull, see Plate 1g, and Plate 3g.

_Comparisons._--For comparisons with _B. m. brunneus_ and _B. m. infernatis_, see accounts of those subspecies.

From _B. m. musculus_, _B. m. pallidus_ differs in: dorsum more olive-gray and brown, less ochraceous on either side of mid-dorsal region; face below eye grayish, not buffy; sides gray with buffy overtone, not creamy with light yellow overtones; venter grayish-white rather than an olive-buff; zygomata more tapering anteriorly; maxillary part of zygoma narrower when viewed from above; external and cranial dimensions smaller.

From _B. m. nigrescens_, _B. m. pallidus_ differs in: dorsum paler, fewer black hairs medially; face paler, less sooty; vibrissae brownish with white tips rather than black with brownish tips; venter paler; dorsal surface of forefeet and hind feet whitish to flesh-colored rather than sooty to dusky-white; tail paler; nasals slightly more attenuated; averaging slightly larger in external and cranial measurements.

_Remarks._--Russell (1952:21) described _pallidus_, on the basis of specimens from the arid Balsas Basin, of Morelos, as pale gray dorsally. After examining the original material from Morelos, I find the dorsal color of _pallidus_ to be much closer to a buffy brown than a pale grayish. Even so, smaller size differentiates _pallidus_ from _musculus_. _B. m. infernatis_, not _B. m. pallidus_, is the most pallid of all named subspecies of _B. musculus_.

_B. m. pallidus_ intergrades to the northwest with _B. m. musculus_, to the northeast with _B. m. infernatis_, and to the southeast with _B. m. nigrescens_.

According to Goodwin (1959:2), _B. m. nebulosus_ (named on the basis of one specimen) differs from _B. m. musculus_ [= _pallidus_] from southern Oaxaca in: darker and longer pelage; larger skull; interorbital region broader and less constricted posteriorly. From _B. m. nigrescens_ and _B. m. brunneus_, _B. m. nebulosus_ differs as follows: pelage longer and softer; skull larger.

Study of specimens of _B. musculus_ from Oaxaca reveals considerable variation in external and cranial measurements as well as color, corresponding to that reported by Goodwin (_loc. cit._). Specimens from higher altitudes average somewhat darker and larger in external and cranial size than those at lower elevations. These differences seem to be microgeographic and not of subspecific rank. Among specimens that I have studied in Oaxaca are several from different localities (KU 63052, an adult male, from 3 mi. W Miahuatlan; KU 68964, an adult male from 3 mi. W Mitla, 6000 ft.; KU 63055, an adult female from 3 mi. S Candelario, 1200 ft.) that, according to Goodwin (_in. litt._) match _nebulosus_ in reported color, size of body and skull (except for the region of the rostrum).