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).
Two of the three specimens (KU 63052 and 63055) are the darkest of a
series in which the palest are inseparable from _B. m. pallidus_.
Goodwin, who kindly compared the three specimens with the type of
_nebulosus_, mentioned (_in. litt._) that the skull of the type has a
slenderer rostrum. Included in the series of skulls of _B. m. pallidus_
from 3 mi. W Mitla are several adults (not seen by Goodwin) with slender
rostra. _B. m. nebulosus_ is judged to be a synonym of _B. m. pallidus_.
Populations of pygmy mice occurring in partially isolated areas of
highland in Oaxaca seem to me to be incipient subspecies.