Chapter IV: Part 4

_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 Potosi 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 Potosi. _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 Tamuin, 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 Maiz, 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 Geronimo, 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 Mexico, as follows: SAN LUIS POTOSI: 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.

_Baiomys taylori paulus_, Miller, Bull. U. S. Nat. Mus., 79:137,
December 31, 1912 (part); Miller, Bull. U. S. Nat. Mus., 128:317,
April 29, 1924 (part); Ellerman, The Families and Genera of Living
Rodents, 2:402, March 21, 1941 (part); Goldman, Smith, Miscl. Coll.,
115:373, July 31, 1951 (part); Hall and Kelson, Univ. Kansas Publs.,
Mus. Nat. Hist., 26:367, December 15, 1952; Goodwin, Bull. Amer.
Mus. Nat. Hist., 102:318, August 31, 1953; Miller and Kellogg, Bull.
U. S. Nat. Mus., 205:511, March 3, 1955 (part); Packard, Proc. Biol.
Soc. Washington, 71:17, April 11, 1958; Packard, Jour. Mamm.,
40:146, February 20, 1959; Hall and Kelson, The Mammals of North
America, 2:659, March 31, 1959 (part).

[_Peromyscus_] _paulus_, Elliot, Field Columb, Mus. Publ.,
95(4):136, July 15, 1904.

_Peromyscus taylori paulus_, Osgood, N. Amer. Fauna, 28:255, April
17, 1909 (part).

_Peromyscus musculus_ [_musculus_], Osgood, N. Amer. Fauna, 28:256,
April 17, 1909 (part).

_Baiomys taylori_ [= _paulus_], Twente and Baker, Jour. Mamm.,
32:121, February 15, 1951.

_Baiomys musculus musculus_, Goldman, Smith. Miscl. Coll., 115:336,
July 31, 1951 (part).

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

_Type._--Adult male, skin and skull; No. 21165, American Museum of Natural History; Rio Sestin, Durango, Republic of Mexico; obtained on April 15, 1903, by J. H. Batty, original number 455.

_Range._--Central Chihuahua south through Durango (west to eastern edge of Sierra Madre Occidental), to Zacatecas and Aguascalientes, thence west into northern and northwestern Jalisco, see Figure 11. Zonal range: Lower Sonoran, approximately the Chihuahua Desert Biotic Province of Goldman and Moore (1945:349). Occurs from 4000 feet 2 mi. ESE Tequila, Jalisco, up to 6700 feet 2 mi. W Minaca, Chihuahua.

_Diagnosis._--Size medium to small for the species; dorsum Buffy Brown to fawn color; dorsal ground color of unworn pelage of adults varying from Buffy Brown in darkest series (especially those from higher altitudes) to Avellaneous with grayish overtones in palest series; worn pelage in mid-dorsal region of adults fawn to grayish; terminal parts of individual hairs buffy, gray basally; guard hairs on dorsum black-tipped, grayish basally; belly Light Gull Gray, distal half of hairs white, proximal half Neutral Gray; hairs in region of throat and chin white to base (some specimens with faint buffy overtones); forefeet dusky below, whitish above; hind feet whitish above, ventral surface whitish to dusky; dorsal and lateral vibrissae black, other vibrissae white. Average and extreme measurements of six adults from the type locality are as follows: total length, 109 (106-117); length of tail, 44.5 (43-48); length of body, 63 (57-69); length of hind foot, 13.1 (12.7-14.0); occipitonasal length, 17.5 (17.4-18.0); zygomatic breadth, 9.3 (9.1-9.5); postpalatal length, 6.6 (6.2-6.9); least interorbital breadth, 3.5 (3.4-3.6); length of incisive foramina, 3.8 (3.6-4.1); length of rostrum, 5.9 (5.7-6.0); breadth of braincase, 8.6 (8.5-8.8); depth of cranium, 6.6 (6.2-6.9); alveolar length of maxillary tooth-row, 3.2 (3.1-3.4); for photographs of the skull, see Plate 2_e_ and Plate 4_f_.

_Comparisons._--For comparisons with _B. t. allex_, _B. t. canutus_, _B. t. ater_, and _B. t. taylori_, see accounts of those subspecies. From _B. t. analogous_, _B. t. paulus_ differs as follows: dorsal color paler having more reddish-brown than blackish-brown tones; venter whitish to buffy, instead of gray to light-gray; tail bicolored (not unicolored), usually having more hairs; hind feet white (not sooty) above. Cranially, _B. t. paulus_ differs from _B. t. analogous_ in: skull slightly smaller in all dimensions; maxillary part of zygoma narrowing and forming oblique angle rather than a near right angle with rostrum; anterior incisive foramina constricted posteriorly; tips of nasals truncate (less rounded).

_Remarks._--J. A. Allen (1903:599) correctly pointed out that young specimens, in first pelage, were gray brown; young adults were darker and more varied with some blackish; adults and old adults were buffy to grayish. The change in color of pelage with increasing age is more pronounced in _paulus_ than in other subspecies of _B. taylori_. Of two males collected on April 12, 1949, one, an adult, is buffy brown, and the other, an old adult with worn pelage, is grayish-brown. In mice in the earlier stages of adulthood, underfur of the dorsum is buffy at the tips and gray basally. With increased wear, the buffy tip is lost. Consequently, mice in the later stages of adulthood are grayish.

_B. t. paulus_ intergrades with _ater_ to the north in Chihuahua (see account of that subspecies), with _analogous_ to the south in Jalisco, and with _allex_ (see account of that subspecies) to the southwest in Nayarit and Jalisco. The zone of intergradation between _paulus_ and _analogous_ in Jalisco approximately borders the Rio Grande de Santiago from the western part of the State to the northwest shore of Lago de Chapala. Nineteen specimens from 2 mi. WNW Lagos de Moreno in northwest Jalisco seem to be intermediate between _paulus_ and _analogous_ in color, averaging slightly grayer than typical _paulus_. The series of 19 is referable to _paulus_ on the basis of cranial characters.

A series of 34 specimens from 3 mi. W La Venta, Jalisco (referable to _paulus_), is indistinguishable in color of pelage from two series of _paulus_ from 5 mi. N Durango, and from 8 mi. NE of Durango, except that the antiplantar surfaces of the hind feet are sooty as in _analogous_. Seemingly, features of color mentioned above as diagnostic of the two subspecies are either present or absent and there is no tendency toward intermediacy in color in the population from 3 mi. W La Venta.

The Rio Grande de Santiago may have acted in the past as a physical barrier reducing gene flow between _allex_ and _paulus_ and in separating completely the two populations for limited periods.

_Specimens examined._--Total 176, all from the Republic of Mexico and distributed as follows: CHIHUAHUA: Rancho Sanignacio, 4 mi. S, 1 mi. W Santo Tomas, 1; El Rosario, 6700 ft., 1; 2 mi. W Minaca, 6900 ft., 11; Balleza, 1[50]. DURANGO: Rosario, 1[51]; type locality, 14[51] (including the type); _San Gabriel_, 2[51]; _Rancho Santuario_, 2[51]; 1 mi. N Chorro, 6450 ft., 1; _8 mi. NE Durango_, 6200 ft., 2; 5 mi. N Durango, 6400 ft., 2. ZACATECAS: Valparaiso, 6500 ft., 10[50]. AGUASCALIENTES: _18 mi. W, 2 mi. S Aguascalientes_, 6000 ft., 1; 16 mi. S Aguascalientes, 5[52]. JALISCO: 1 mi. NE Villa Hidalgo, 6500 ft., 1; 2 mi. WNW Lagos de Moreno, 6370 ft., 19; _2 mi. ESE Tequila_, 4000 ft., 11; _3 mi. W La Venta_, 33, 1[53]; _12 mi. W Guadalajara_, 3[54]; _Atemajac_, 12[50]; 4 mi. W Guadalajara, 5100 ft., 3; _2 mi. N, 1/2 mi. W Guadalajara_, 11; 2 mi. NW Magdalena, 4500 ft., 7[50]; _1 mi. N Tala_, 4400 ft., 3; 3 mi. W Tala, 4300 ft., 18.

_Marginal records._--CHIHUAHUA: Rancho Sanignacio, 4 mi. S, 1 mi. W Santo Tomas; El Rosario; Balleza. DURANGO: Rosario, 6700 ft.; 1 mi. E Zarca (Blossom and Burt, 1942:1); 1 mi. N Chorro, 6450 ft. ZACATECAS: Valparaiso, 6500 ft. AGUASCALIENTES: 1 mi. N Chicalote (Blossom and Burt, 1942:4). JALISCO: 2 mi. WNW Lagos de Moreno, 6370 ft.; 4 mi. W Guadalajara, 5100 ft.; 3 mi. W Tala, 4300 ft.; 2 mi. NW Magdalena, 4500 ft. DURANGO: 5 mi. N Durango, 6400 ft.; type locality. CHIHUAHUA: 2 mi. W Minaca, 6900 ft.

[50] United States National Museum (Biol. Surv. Collections).

[51] American Museum of Natural History.

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

[53] The Museum, Michigan State Univ.

[54] Univ. Michigan, Museum of Zoology.

=Baiomys taylori subater= (V. Bailey)

_Peromyscus taylori subater_, V. Bailey, N. Amer. Fauna, 25:102,
October 24, 1905; Lyon and Osgood, Bull. U. S. Nat. Mus., 62:139,
January 15, 1909; Osgood, N. Amer. Fauna, 28:255, April 17, 1909;
Elliot, Check-List Mamm. N. Amer. Continent, West Indies and
Neighboring Seas, Suppl., Amer. Mus. Nat. Hist, p. 44, January 8,
1917.

_Baiomys taylori subater_, Miller, Bull. U. S. Nat. Mus., 79:136,
December 31, 1912; Miller, Bull. U. S. Nat. Mus., 128:317, April 29,
1924; Anthony, Field Book of North American Mammals, p. 348, 1928;
Baker, Jour. Mamm., 21:223, May 14, 1940; Ellerman, The Families and
Genera of Living Rodents, 2:402, March 21, 1941; Blair, Jour. Mamm.,
22:378, November 14, 1941; Poole and Schantz, Bull. U. S. Nat. Mus.,
178:259, March 6, 1942; Blair, Jour. Mamm., 23:196, May 14, 1942;
Blair and Blossom, Contrib. Lab. Vert. Biol., Univ. Michigan, 40:1,
March, 1948; Miller and Kellogg, Bull. U. S. Nat. Mus., 205:511,
March 3, 1955; Hall and Kelson, The Mammals of North America, 2:659,
March 31, 1959.

_Baiomys taylori_ [= _subater_], Taylor and Davis, Texas Game, Fish
and Oyster Comm. Bull., 27:56, August, 1947 (part).

_Type._--Subadult female, skin and skull; No. 32616/44539 U. S. Nat. Mus. (Biol. Surv. Coll.); Bernard Creek, near Columbia, Brazoria County, Texas; obtained on February 25, 1892, by W. Lloyd, original number 1122.

_Range._--Southeastern Texas, north of Matagorda Bay west to Lavaca County, north to Brazos and Walker counties thence east to Jefferson County, see Figure 11. Occurs from near sea level in Brazoria and Galveston counties, up to 500 feet in western part of range. Zonal range: Humid division of lower Austral (the western part of the Austroriparian Biotic Province of Dice, 1943:18-21).

_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.

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.]

Photographs of skulls (_a-g_) in dorsal view of _Baiomys_. x 2.

_a._ _B. t. analogous_, [F] ad., 120265, USNM, Zamora, Michoacan.
_b._ _B. t. ater_, [F] ad., 15056, UI, 1-1/2 mi. ENE Greaterville,
Arizona.
_c._ _B. t. canutus_, [F] ad., 62076, KU, 1 mi. S Pericos, Sinaloa.
_d._ _B. t. fuliginatus_, [F] ad., 36771, KU, type locality.
_e._ _B. t. paulus_, [F] ad., 40032, KU, 18 mi. W, 2 mi. S
Aguascalientes.
_f._ _B. t. subater_, [F] ad., 44543, USNM, type locality.
_g._ _B. t. taylori_, [F] ad., 57944, KU, 5 mi. E San Antonio, Texas.
_h._ Photo. of captive [M] _B. t. taylori_, 25 mi. E Austin, Texas.
x 1.]

Photographs of skulls in ventral 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.]

Photographs of skulls in ventral view of _Baiomys_. x 2.

_a._ _B. t. allex_, [F] ad., 45453, USNM, Colima, Colima.
_b._ _B. t. analogous_, [F] ad., 120265, USNM, Zamora, Michoacan.
_c._ _B. t. ater_, [F] ad., 15056, UI, 1 mi. ENE Greaterville, Arizona.
_d._ _B. t. canutus_, [F] ad., 62076, KU, 1 mi. S Pericos, Sinaloa
_e._ _B. t. fuliginatus_, [F] ad., 36771, KU, type locality.
_f._ _B. t. paulus_, [F] ad., 40032, KU, 18 mi. W, 2 mi. S
Aguascalientes.
_g._ _B. t. subater_, [F] ad., 44543, USNM, type locality.
_h._ _B. t. taylori_, [F] ad., 57944, KU, 5 mi. E San Antonio, Texas.]

_Remarks._--This subspecies retains its chief diagnostic character, blackish mid-dorsal region, throughout nearly all parts of its range. Specimens from the general area of Matagorda Bay and Lavaca County grade into _taylori_ in characters of color and crania. The Colorado and Brazos rivers seemingly serve as barriers reducing gene flow between _taylori_ and _subater_. These rivers may well have been important factors in the origin and the limitation of these two seemingly closely-related subspecies.

_Baiomys taylori subater_ is not differentiated in color of pelage and characters of crania from _B. t. taylori_ to the same degree that _B. t. paulus_ is differentiated from _B. t. analogous_, or that _B. t. taylori_ is differentiated from several of the other subspecies of _Baiomys taylori_. _B. t. subater_ probably is a more recent occupant of the area in which it now lives than is the case with any other one of the subspecies of _taylori_. Sufficient time probably has not elapsed to allow for formation of more distinctive phenotypic patterns.

_Specimens examined._--Total 65, all from TEXAS and distributed as follows: _Brazos County_: 1/2 mi. NW College Station, 1[55]; _3 mi. W College Station_, _1 mi. W Easterwood Airport_, 1[55]; _College Station_, 1[55]. _Walker County_: Huntsville, 1[55]. _Hardin County_: Sour Lake, 1[57]. _Jefferson County_: 7 mi. S Labelle, 10. _Harris County_: 6 mi. NE Crosby, 1[56]. _Colorado County_: _10 mi. N Eagle Lake_, 1[55]; _9 mi. N Eagle Lake_, 1[55]; 2 mi. W Eagle Lake, 1; _Eagle Lake_, 1[55], 5. _Fort Bend County_: Richmond, 4[57]. _Galveston County_: _Texas City_, 6[58]; Virginia Point, 1[57]. _Brazoria County_: _Austin Bayou near Alvin_, 2[57]; 14 mi. SSE Alvin, 2[59]; type locality, 7[57] (including the type). _Lavaca County_: 4 mi. W Hallettsville, 1[55]; _1 mi. SW Hallettsville_, 3[55]; _13.7 mi. SW Hallettsville_, 2[55]; 4 mi. NE Yoakum, 11.

_Marginal records._--TEXAS: Huntsville; Sour Lake; 7 mi. S La Belle; Virginia Point; 14 mi. SSE Alvin; type locality; 4 mi. NE Yoakum; 4 mi. W Hallettsville; 1/2 mi. NW College Station.

[55] Texas A & M, Cooperative Wildlife Research Collection.

[56] Carnegie Museum.

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

[58] Los Angeles County Museum.

[59] American Museum of Natural History.

=Baiomys taylori taylori= (Thomas)

_Hesperomys_ (_Vesperimus_) _taylori_ Thomas, Ann. Mag. Nat. Hist.,
ser. 5, 19:66, January, 1887.

_Baiomys taylori_ [_taylori_], Mearns, Bull. U. S. Nat. Mus.,
56:381, April 13, 1907; Stickel and Stickel, Jour. Mamm., 30:141,
May 23, 1949.

Baiomys taylori taylori, Miller, Bull. U. S. Nat. Mus., 79:136,
December 31, 1912; Miller, Bull. U. S. Nat. Mus., 128:317, April 29,
1924; Anthony, Field Book of North American Mammals, p. 327, 1928;
Ellerman, The Families and Genera of Living Rodents, 2:402, March
21, 1941; Taylor and Davis, Texas Game, Fish and Oyster Comm. Bull.,
27:56, August, 1947 (part); Blair, Texas Jour. Sci., 2:104, March
31, 1950; Goldman, Smith. Miscl. Coll., 115:373, 426, July 31,
1951; Baker, Univ. Kansas Publs., Mus. Nat. Hist., 5:212, December
15, 1951; Blair, Texas Jour. Sci., 4:242, June 30, 1952; Hooper,
Occas. Papers, Univ. Michigan, Mus. Zool., 544:7, March 25, 1953;
Dalquest, Louisiana State Univ. Studies (Biol. Sci. Ser.), 1:155,
December 28, 1953 (part); Blair, Adv. in Genetics, 5:10, January 27,
1954; Miller and Kellogg, Bull. U. S. Nat. Mus., 205:511, March 3,
1955; Baker, Univ. Kansas Publs., Mus. Nat. Hist., 9:273, June 15,
1956; Packard, Proc. Biol. Soc. Washington, 71:17, April 11, 1958;
Hall and Kelson, The Mammals of North America, 2:659, March 31, 1959
(part).

_Cricetus_ (_Vesperimus_) _taylori_, Thomas, Proc. Zool. Soc.
London, 68:446, November 20, 1888.

_Sitomys taylori_, Merriam, Proc. Biol. Soc. Washington, 7:170,
September 29, 1892.

_Sitomys_ (_Baiomys_) _taylori_, True, Proc. U. S. Nat. Mus.,
16(972):758, February 7, 1894; J. A. Allen, Bull. Amer. Mus. Nat.
Hist., 6:181, May 31, 1894.

_S._ [_itomys_] _taylori_, Rhoads, Proc. Acad. Nat. Sci.
Philadelphia, 46:256, October, 1894.

_Peromyscus_ (_Baiomys_) _taylori_, J. A. Allen, Bull. Amer. Mus.
Nat. Hist., 8:65, April 22, 1896.

[_Peromyscus_] _taylori_, Trouessart, Cat. Mamm., 1:517, 1898.

_Peromyscus taylori_ [_taylori_], Elliot, Field Columb. Mus. Publ.,
105(4):135, July 1, 1905; V. Bailey, N. Amer. Fauna, 25:101, October
24, 1905; Elliot, Field Columb. Mus. Publ., 115(8):203, 1907;
Osgood, N. Amer. Fauna, 28:253, April 17, 1909.

_Type._--Adult male, skin and skull; No. 87.11.24.1, British Museum, Natural History; San Diego, Duval County, Texas; obtained by William Taylor.

_Range._--North-central to southeastern Texas, excluding the coastal plain north of the region of Matagorda Bay, thence south into the southern part of Tamaulipas and west into Coahuila and Nuevo Leon, see Figure 11. Occurs from near sea level in Texas up to 1500 feet in Coahuila. Zonal range: mostly Lower Austral (in Mexico and southeastern half of Texas, the Tamaulipas Biotic Province of Goldman and Moore, 1945:349, and Blair, 1952:230).

_Diagnosis._--Size medium for the species; dorsum grayish in freshly taken specimens to Hair Brown in preserved specimens; individual guard hairs of dorsum black-tipped, grayish basally, underfur black-tipped with a subterminal band of olive-buff; sides of body pale-grayish near venter, individual hairs buffy proximally, grayish basally; belly pale grayish, individual hairs white-tipped, Pale Neutral Gray basally; throat and chin colored as is belly; forefeet and hind feet sooty-gray dorsally, sparsely-haired ventrally, thus appearing flesh-colored; tail unicolored gray to sooty-gray. Average and extreme cranial measurements of 22 adults from 6 mi. SW San Geronimo, Coahuila, are as follows: occipitonasal length, 18.0 (17.4-19.0); zygomatic breadth, 9.6 (9.2-10.2); postpalatal length, 6.5 (5.9-7.1); least interorbital breadth, 3.6 (3.3-3.8); length of incisive foramina, 4.0 (3.6-4.3); length of rostrum, 6.1 (5.7-6.7); breadth of brain case, 8.8 (8.5-9.1); depth of cranium, 6.5 (6.0-7.0); alveolar length of maxillary tooth-row, 3.1 (3.0-3.3). Average and extreme external measurements of 19 adults from 6 mi. SW San Geronimo are as follows: total length, 102.2 (95-115); length of tail vertebrae, 39.4 (21-46); length of body, 62.8 (53-76); length of hind foot, 14.0 (12-15); length of ear from notch, 10.7 (10-12); for photographs of skull, see Plate 2_g_, and Plate 4_h_.

_Comparisons._--For comparisons with _B. t. subater_, _B. t. analogous_, and _B. t. fuliginatus_, see accounts of those subspecies. From _B. t. paulus_, found to the southwest, _B. t. taylori_ differs as follows: dorsum grayish rather than fawn-colored; hairs on dorsal parts of forefeet and hind feet sooty-gray (not white to white-brown); venter gray to Light Drab-Gray, rather than whitish with gray overtones; tail unicolored instead of bicolored; skull averaging slightly larger over-all; maxillary part of zygoma forms right angle with rostrum rather than obtuse angle; incisive foramina extending posteriorly to anterior plane of first upper molars instead of to a transverse plane at middle of right and left first upper molars; bullae less inflated; interorbital region broader relative to length of skull; rostrum sloping gently from frontonasal suture to anterior tip of nasals rather than declining abruptly from frontonasal suture to anterior tip of nasals.

_Remarks._--The geographic range of _taylori_ is relatively large, and the subspecies is locally variable. Nevertheless, none of the external and cranial measurements of specimens assigned to this subspecies differs significantly from the corresponding measurements of material from the type locality and adjacent areas in southeastern Texas. In southeastern Texas, south of the Guadalupe River, south to the coastal plain of Tamaulipas, this subspecies differs in color (being paler) from _B. t. subater_ with which _taylori_ might be confused. The foothills of the Sierra Madre Oriental in western Tamaulipas, north through Nuevo Leon and Coahuila, seem to mark the southwestern limit of the range assignable to _taylori_.

On December 27, 1958, a specimen, KU 81552, was obtained 3 mi. N Bowie, Montague County, Texas. This record station extends the known range of _B. taylori_ 65 miles northward from the previous northernmost locality, listed by Hunsaker, Raun, and Swindells (1959:447). Two specimens, KU 81553 and 81554, were collected by the author 2 mi. NE Cedar Hill, Dallas County, Texas, on October 31, 1958. These two specimens, plus the single specimen from Bowie County are all paler with more buffy bellies than either _B. t. taylori_ or _B. t. subater_. They may represent an incipient subspecies. I tentatively assign them to _B. t. taylori_ because of the pale rather than dark (like _B. t. subater_) pelage. Additional specimens are needed from these areas and from the hiatus between the ranges of _B. t. taylori_ and _B. t. subater_ the better to understand the manner in which these two subspecies intergrade.

Among named subspecies of _Baiomys taylori_, _B. t. taylori_ most closely resembles _B. t. subater_ to the north in Texas. Nine specimens examined from Yoakum are intergrades between _taylori_ and _subater_. These specimens have the sooty dorsal color of _subater_, but ventrally are inseparable from topotypes of _taylori_. In length of body and tail, specimens from Yoakum are like _subater_, but in length of hind foot, they are intermediate between the two subspecies. Cranially, they are like _subater_. When all characters are considered, the specimens are best referred to _subater_. Bailey (1905:103) suggested that specimens from the southern part of the range, which he ascribed to _subater_, tended to a more grayish color than topotypes of _subater_, therefore, grading into _taylori_. The zone of intergradation runs from Matagorda Bay northwest through Lavaca County, thence north to the Colorado River, and closely follows the boundary between the Lower Austral and Humid Division of Lower Austral Life-zone as plotted by Bailey (_loc. cit._). Findley (1955:44) pointed out that where two life-zones meet, the resulting populations of shrews are mostly intergrades. Such is the case between these two subspecies of _Baiomys taylori_ in an area where life-zones might seem less important than in the mountainous west.

In the southern part of the range of _taylori_, intergradation occurs between _B. t. taylori_ in western Tamaulipas and _B. t. fuliginatus_ in the mountains of San Luis Potosi.

Dalquest (1953:156) found no indication of intergradation between the two species, _B. taylori_ and _B. musculus_, in San Luis Potosi. After examination of specimens from San Luis Potosi, I am in agreement that they are all referable to the species _taylori_.

_Specimens examined._--Total 435. TEXAS: _Montague County_: 3 mi. N Bowie, 1. _Dallas County_: 2 mi. NE Cedar Hill, 2. _Travis County_: 8 mi. NW Austin, 2[60]; _Austin_, 2[60]; _4 mi. E Austin_, 4[60]; _5 mi. E Austin_, 3[60]; _6 mi. E Austin_, 16[60], 1; _7 mi. E Austin_, 1[60]; _15 mi. E Austin_, 1[60]; _4 mi. S Austin_, 1[60]. _Bastrop County_: 25 mi. E Austin, 2. _Kendall County_: Boerne, 1[61]. _Bexar County_: _1 mi. N Randolph Field_, 3[64]; _5 mi. ENE_ (_on U. S. Highway 81_) _San Antonio_, 1; _3 mi. NE San Antonio_, 1; San Antonio, 26[61], 11[62], 1[63]; _5 mi. E San Antonio_, 11; _4-1/2 mi. E Sayers_, 3. _Gonzales County_: 7 mi. S Luling, 2[60]. _Wilson County: 4 mi. W LaVernia_, 3; 12 mi. W Floresville, 1. _Atascosa County_: 9 mi. SW Somerset, 1. _Goliad County_: 8 mi. NE Goliad, 1[60]. _Bee County_: Beeville, 1[61]. _Aransas County_: Aransas (Wildlife) Refuge, 1[65]; _5 mi. E Copana Bay_, 1[65]; _4.6 mi. NE Rockport_, 5[60]; _4.5 mi. NW Rockport_, 2[60]; 3 mi. N, 2 mi. E Rockport, 4; _Rockport_, 1[60], 1[61], 1[63]; _1-1/2 mi. SW Rockport_, 1[60]; _2 mi. SW Rockport_, 2[60]; _13.4 mi. SW Rockport_, 1[60]; _14 mi. SW Rockport_, 1. _San Patricio County_: Welder Wildlife Refuge, 7. _Duval County_: type locality, 2[61], 1[66]. _Nueces County_: Corpus Christi (south Nueces Bay), 1[64] (Cleveland Mus. Coll.). _Kleberg County_: 2 mi. S Riviera, 3[65]. _Brooks County_: 3 mi. S Falfurrias, 2[65]. _Hidalgo County_: 6 mi. S McAllen, 17[60]. _Willacy County_: 28 mi. E Raymondville, 10[65]. _Cameron County_: Brownsville, 31[61], 23[62], 5[64]. COAHUILA: 6 mi. SW San Geronimo, 32. NUEVO LEON: Santa Catarina, 1[61]; 14 mi. N Monterrey, 1950 ft., 2[67]; Monterrey, 1[61]; 20 km. N General Teran, 3[64]. TAMAULIPAS: _Near Headwaters Rio Sabinas, 8 km. W, 10 km. N El Encino_, 400 ft., 1; Camargo, 5[61]; Charco Escondido, 20 mi. S Reynosa, 3[67]; Matomoras, 5[61]; _Ejido Santa Isabel, 2 km. W Inter-American Highway_, 2000 ft., 7; Hidaglo, 7[61]; _Hda. Station Engracia_, 4[63]; 4 mi. N La Pesca, 1; 29 mi. N Ciudad Victoria, 1[67]; Ciudad Victoria, 6[61], 3; Jaumave, 2400 ft., 6[64], 10; Sierra de Tamaulipas, 3[64]; _25 mi. N El Mante, 3 km. W Inter-American Highway_ (_on Rancho Pano Ayuctle_), 300 ft., 4; _6 mi. N Gomez Farias_ (_on Rancho Pano Ayuctle_), 1; _5 mi. NE Gomez Farias_, 12[64], 1[62]; 70 km. (by highway) S Ciudad Victoria, 2 km. W El Carrizo, 5[62], 2; Antigua Morelos, 5[64]; _6 mi. N, 6 mi. W Altamira_, 31; _5 mi. N, 5 mi. W Altamira_, 4; _Alta Mira_ (_Altamira_), 2[61]; 1 mi. S Altamira, 6; _10 mi. NW Tampico_, 1. SAN LUIS POTOSI: Ebano, 5[68]; _4 km. NE Ciudad Valles_, 1; Ciudad Valles, 1; _3 km. W Tamuin_, 1[68]; _Tamuin_, 6[68]; _Pujal_, 300 m., 1[64]. VERACRUZ: Tampico Alto, 50 ft., 1; Potrero Llano, 350 ft., 1; Ozulama, 2; Cerro Azul, 350 ft., 1.

_Marginal Records._--TEXAS: 3 mi. N Bowie; 2 mi. NE Cedar Hill; 25 mi. E Austin; 7 mi. S Luling; 8 mi. NE Goliad; Aransas (Wildlife) Refuge; 3 mi. N, 2 mi. E Rockport; Corpus Christi (South Nueces Bay); 2 mi. S Riviera; 28 mi. E Raymondville; Brownsville. TAMAULIPAS: Matomores; 4 mi. N La Pesca; 1 mi. S Altamira. VERACRUZ: Tampico Alto; Ozulama; Cerro Azul; Potrero Llano. SAN LUIS POTOSI: Ciudad Valles. TAMAULIPAS: Antigua Morelos; 70 km. S Ciudad Victoria, 2 km. W El Carrizo; Jaumave; Hidalgo. NUEVO LEON: 20 km. N General Teran; Santa Catarina. COAHUILA: 6 mi. SW San Geronimo. TEXAS: 9 mi. SW Somerset; Boerne; 8 mi. NW Austin.

[60] Coll. University of Texas.

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

[62] American Museum of Natural History.

[63] Chicago Natural History Museum.

[64] Univ. Michigan, Museum of Zoology.

[65] Texas A & M Coop. Wildlife Res. Coll.

[66] Carnegie Museum.

[67] Univ. California, Mus. Vert. Zool.

[68] Museum of Natural History, Louisiana State University.

EVOLUTION AND SPECIATION

The history of the genus dates back to the early late Pliocene, but morphological change since then has been slight insofar as can be judged from lower jaws. _Baiomys_ seems to have been relatively conservative also in types of habitat occupied.

According to Wilson (1937:59), the late Pliocene was a time of decided expansion of myomorph rodents, more particularly cricetines. Furthermore, at this time, the climate in the interior basin of southwestern North America presumably was becoming arid, if we can judge from the spread of elements of the Madro-Tertiary flora. Axelrod (1950:266) points out that the drier, continental climate initiated in the early Tertiary probably had its culmination in middle Pliocene time. Some floras of early late Pliocene of the southwestern United States reflect a climate slightly cooler and more moist than the climates of the middle Pliocene. However, late Pliocene times reflect an arid climate. The flora of the southwestern interior basin of North America in early late to late Pliocene was intermediate between the previous grassland floras of the middle Pliocene and the savannah flora of upper Pliocene. Axelrod (_loc. cit._) suggests that this intermediate flora of the interior basin of southwestern North America resulted from the folding of the Cascades and uplifting of the Sierra Nevada and Peninsular ranges to the south. The development of these mountains produced greater aridity to the lee of the mountains, thus accounting for the grassland-savannah flora. Pygmy mice probably originated in that time, I judge in Mexico, and moved northward and southward in a grassland-savannah habitat that seemingly existed as far north as what is now Meade County, Kansas (where the Sawrock fauna lived). Further evidence for occupancy of a grassland-savannah habitat by ancestral pygmy mice stems from the distribution of the living species, _B. taylori_, that at present occupies territory adjacent to parts of the Sonoran and Chihuahuan deserts. _B. taylori_ seems to be morphologically more specialized for life in an arid grassland than was _B. sawrockensis_.

The geographic range of ancestral pygmy mice possibly extended farther south in late Pliocene time than the range of _B. musculus_ does now. Anyhow, _B. sawrockensis_ of the early late Pliocene dwelt in a more mesic type of habitat than _B. musculus_ does, and such habitat may have existed from the Pacific lowlands of Central America to the Caribbean lowlands of northern South America (see Duellman, 1958:136, and Dunn, 1940:156) during late Pliocene times. An ancestral stock of hesperomine mice, not greatly different from _Baiomys_, may have emigrated from the North American continent into South America across the continuous land connection, which Simpson (1950:395) suggests was formed in the Chapadmalalan age (= Blancan age of North American terminology). The length of time of interchange of genes between northern and southern populations of mice across the Central American land connection probably was brief. Duellman (_op. cit._:129) pointed out that once the Panamanian portal was closed, the warm counter equatorial current, El Nino, combined with the uplifting of the Andes, began to produce heavy rain forests in Central America and northern South America in late Pliocene or early Pleistocene times. These forests presumably isolated the stock in North America from that in South America where the latter probably evolved rapidly into kinds that differed from one another and from _Baiomys_ in shape of body, type of pelage, and shape of skull. Internal structures such as hyoid apparatus, auditory ossicles, and baculum remained almost unchanged, as for example in _Calomys_ now living in South America. The present resemblance in internal morphological features between it and _Baiomys_, I judge, reflects taxonomic relationships more accurately than do shape and conformation of body and skull that seem to respond more rapidly to external environmental changes. The cranial characters distinguishing _Baiomys musculus_ from _Calomys laucha_ are as follows: posterior lacerate foramina between second, rather than first, upper molars; parapterygoid fossa shallower; mesopterygoid fossa as wide or wider, instead of narrower, than parapterygoid processes; burr for attachment of superficial masseter muscle hypertrophied instead of well-developed. In other cranial characters studied, the two genera closely resemble each other. Such similarities of crania between _Calomys_ and _Baiomys_ may reflect convergence, but the total of internal and external morphological characters shared, I think reflects true relationships.

_Peromyscus_ has a large number of living and extinct species and exhibits a wide range of morphological variation, whereas _Baiomys_ has a small number (7) of species and exhibits a narrow range of morphological variation. The small number of known species of pygmy mice suggests their conservatism in elaboration of morphological characters. Possibly this is because the habitat, or even the ecological niche, occupied in geological time by these mice was restricted, geographically and in kind. If the habitat of the pygmy mice oscillated between savannah and arid grassland, then an hypothesis can be made possibly accounting for the origin of species of these mice. My idea is that the geographical distribution of _Baiomys_ today reflects a predilection on the part of these mice for a relatively uniform warm climate. Therefore, in the past, in times of warmer continental climate, these mice moved toward favorable habitat northward from an area in central and northern Mexico. In cooler periods, the mice moved southward as habitats to the north became unfavorable.

Dr. W. B. Davis (_in. litt._) informs me that _B. taylori_ was uncommon in Brazos County, Texas, approximately 15 years ago, and suggests that the abundance there now of this mouse and my taking it in 1958 northward nearly to the southern border of Oklahoma reflects a definite movement northward. Movement in the same direction in late years has been suggested for the nine-banded armadillo and the hispid cotton rat (Hall, 1959:373) that are associated with warm climates to the south. These movements possibly reflect only minor fluctuations of climate, but in a long period of warmth movements northward would be expected to be pronounced and extensive.

Extinct species of _Baiomys_ may have originated as a result of extension northward of the geographical range and subsequent retreat southward of the northern populations, as follows: (1) the range of the genus moved northward in a warm period; (2) in cooler times, most of the mice in the north disappeared and only isolated colonies remained in small patches of remaining habitat still favorable to the mice; (3) the small populations of isolated pygmy mice after a time changed through mutations, recombinations and subsequent selection to a degree that prevented crossbreeding once populations from the south again moved northward and came in contact with previously isolated stocks; (4) then competition caused further divergence in morphological characters. Such an hypothesis would account for the morphological differences between the extinct _B. kolbi_ and _B. rexroadi_. The extinct _B. brachygnathus_, presumably a dweller of a xerophytic grassland, may have had its origin from a _B. minimus_-like stock in the manner outlined.

FORMATION OF THE RECENT SPECIES

The morphological difference between the extinct _B. minimus_ and the living _B. musculus_ is not great, and musculus seems to be the product of the _B. sawrockensis-B. minimus_ line of development. Morphological characters of the parental stock of the two living species, _musculus_ and _taylori_, may have been intermediate between those of _B. minimus_ and those of _B. musculus_. The principal part of the range of _Baiomys_ today is in Mexico, and probably was there through much of Pleistocene time. Extension northward of the species and retreat southward of those northern populations of pygmy mice would not only have left isolated populations in the north, but would have allowed the mice that retreated south to share a common gene pool. Therefore, populations of pygmy mice occurring to the south in central Mexico might be expected to maintain a relatively high degree of heterozygosity in morphological and behavioral characters. The occurrence of any physical or biotic barrier that would have separated this homogeneous group would be conducive to speciation. There is evidence that a barrier occurred in the Pleistocene in central Mexico sufficient to separate the supposed interbreeding, relatively homogeneous populations of pygmy mice. According to Sears (1955:529) and De Terra _et al_. (1949:51), parts of the higher regions in the Valley of Mexico, and the transverse volcanic zone in central Mexico were glaciated. On the mountain Ixtaccihuatl, De Terra (_op. cit._:52) found evidence of four marked advances of ice, from oldest to youngest, as follows: Salto, ice advanced to 3100 meters; Xopano, ice at 3200-3300 meters; Trancas, ice to 3400 meters; Ayolotepito, ice to 4350 meters. The Salto advance is correlated by De Terra (_loc. cit._) with the Iowan glacial period. The advance of ice down the mountain sides in the transverse volcanic zone was accompanied by cool moist climates or pluvial periods. Such climates probably altered habitat formerly suitable for _Baiomys_. There is no record of _Baiomys_ known to me exceeding 8000 feet in elevation, although the lower edge of the ice on Ixtaccihuatl is at approximately 15,300 feet (4600 meters, Sears, _loc. cit._). Presumably, the advance of ice down the mountains forced the pygmy mice to move to lower altitudes. Pluvial conditions possibly rendered the habitat even at lower altitudes uninhabitable for the mice, with the result that none continued to live in the transverse volcanic zone, but only north and south thereof. Long-continued separation of these northern and southern segments allowed species formation to occur. As climatic and habitat conditions became more favorable in central Mexico, the two species moved back toward each other, and eventually their geographic ranges overlapped.

An analysis of external and cranial characters of pygmy mice (see Figure 12) reveals that both species are essentially largest to the north and smallest to the south. There are exceptions to this cline in both species. For example, _B. taylori analogous_ is a large subspecies; it lives allopatrically in the southern part of the range of the species. _B. musculus pallidus_ is not the largest subspecies; it lives allopatrically in the northern part of the range of the species. In west-central Mexico, where the two species are sympatric, _B. taylori_ is smaller than elsewhere and _B. musculus_ is larger than elsewhere. _B. t. analogous_ lives in the mountains of the transverse volcanic zone in central Mexico. Its large size may be a result of the cooler climate in the mountains. _B. t. allex_, the smallest subspecies, lives sympatrically with _B. musculus musculus_ at lower elevations in west-central Mexico. The small size of _allex_ could be a result of the warmer climate of the lower elevations. _B. m. pallidus_, at lower elevations in southern Oaxaca, is smaller than other subspecies of _musculus_ to the south at higher elevations. _B. m. musculus_ lives at low elevations along the coast of west-central Mexico. Unlike _B. m. pallidus_, _B. m. musculus_ is large at lower elevations. It occurs sympatrically with _B. t. allex_. It is my idea that during the period of separation, when the two species were evolving, larger subspecies evolved to the north or at higher altitudes where climates were cooler; smaller subspecies evolved to the south or at lower elevations; the two cognate species, _musculus_ and _taylori_, made contact at lower elevations where individuals of _taylori_ may have been smallest, but individuals of _musculus_ were not the largest of the species. The differences, therefore, between the two species in their initial contact probably were slight. Hybrids, if they occurred, were probably inviable, sterile, or ill-suited for occupancy of the habitat of either of the parental stocks. The occurrence of hybrids, therefore, would result in what geneticists call "gamete wastage," and any further divergence in the parental stock, either in external characters (size and shape of body and head), or behavior, useful in recognition of species, would be favored by natural selection (see Dobzhansky, 1951:225; and Koopman, 1950:147). The two species seem to have diverged more in external characters where they occur together than in areas where they live separately (see Figure 12). The two species could be confused if a sample of adults of _taylori_ from 7 mi. S La Belle, Jefferson County, Texas, were compared to a sample of adults of _musculus_ from Tehuantepec, Oaxaca (see Figure 12). No confusion in species identity would arise, however, if a sample of adults was taken from the area where the two species live together (see Figure 12). Brown and Wilson (1956:49) pointed out that where two closely related species occur together, characters (morphological, ecological, physiological, or behavioral) of each species are easily distinguished. However, where the two species are allopatric, the two closely related species so resemble one another that the species are not easily distinguished. This phenomenon has been called "character displacement" by Brown and Wilson (_loc. cit._).

In the area where the two species of pygmy mice occur together, there seems to be a disparity in numbers between them. Hooper (1952a:91) has recorded the collection of both _B. musculus_ and _B. taylori_ in a single trap line. A series of pygmy mice collected from San Gabriel, Jalisco, contained one _taylori_ and 33 _musculus_; another sample from La Resolana, Jalisco, had a ratio of 25 _taylori_ to 6 _musculus_. The disparity in numbers where the two species occur together has been further substantiated by collections of the University of Kansas. Possibly this disparity in numbers is a result of interspecific competition. Hooper (_op. cit._:90) pointed out that where the range of _B. musculus_ (typical of arid tropical lowlands) meets that of _B. taylori_ (typical of arid temperate highlands), the two geographic ranges interdigitate with parts of the range of _musculus_ extending into the highlands and parts of the range of _taylori_ extending into the lowlands. In the lowlands, _musculus_ may be better adapted to environmental conditions and, therefore, more successful in competition with _taylori_ for available habitat. The reverse situation may exist in the highlands. Also, the fact that _musculus_ is more of a diurnal animal than is _taylori_ may account for the difference in numbers of individuals of the two species taken in trap lines. Many collectors set their traps in late afternoon or evening and retrieve them in early morning. Such a schedule might not yield many _musculus_. If interspecific competition does occur in the area where the two species occur, any change in habits or microhabitat by either species that would reduce this competition would be favored by natural selection (see Mayr, 1949:518; Lack, 1944:262-263; and Brown, 1958:154-155). Brown (_op. cit._:154), as I understand him, pointed out (taking account of Gause's principle) that when two species having similar ecological valences move into the same niche in the same locality, one of three things must eventually happen: (_a_) the two species occupy different geographic ranges; (_b_) they compete and one is eventually eliminated; (_c_) the two species, because of differentiation or specialization, exploit different aspects of the niche. In _Baiomys_, (_c_) seems to apply. Natural selection probably would favor a continuation of diurnal activity in _musculus_ and nocturnal activity in _taylori_, thereby preventing frequent meeting of the two species.

AREAS OF PRESENT DIFFERENTIATION

In both species of _Baiomys_, the most distinct subspecies, _B. t. allex_ and _B. m. musculus_, occur in the area where the two species are sympatric. Seven subspecies, or 44 per cent, occur either in or adjacent to the transverse volcanic zone. This area is the major area of active differentiation. Incipient subspecies are also evident in these areas. A secondary area of differentiation is indicated within the range of _B. musculus_ in Guatemala, El Salvador and Honduras. Three subspecies occur in this area (_grisescens, handleyi_ and _nigrescens_) and incipient subspeciation is in evidence there.

ZOOGEOGRAPHIC POSITION

Hooper (1949:25) regards _Baiomys_ as a member of the rodent fauna of the arid, western Sonoran region, whereas Hershkovitz (1958:609) suggests that _Baiomys_ is a nearctic-neotropical varicant (a kind that occurs in contiguous zoogeographic regions without our knowing in which region the taxon originated). The findings from my study do not contradict either of the above suggestions. Because of the close resemblance of _Baiomys_ to certain hesperomine mice of South America, it is postulated that _Baiomys_, in more primitive form than now, occurred farther south in past times than it does now. Fossils show that primitive stocks of the genus in late Pliocene or early Pleistocene times occurred also north of the present range of the genus. The belt in west-central Mexico between nearctic and neotropical regions is the current center of distribution of the genus and probably has been for a considerable time.

FIG. 12. Averages of the occipitonasal lengths of skulls of adults
at 19 localities of occurrence (solid symbols) of _Baiomys taylori_,
and at 17 localities of occurrence (open symbols) of _Baiomys
musculus_. Note that the occipitonasal length decreases from north
to south in each of the two species, and that in the region where
the two species occur together, west-central Mexico, _B. taylori_
is smallest and _B. musculus_ is largest. Average, extremes, number
of specimens averaged (in italic type), and name of locality, from
north to south for each species, are as follows:

_Baiomys taylori_

18.0 (17.5-18.6) _15_, 9-1/2 mi. W New Mexico state line, Ariz.
18.9 (18.2-19.4) _6_, 7 mi. S. La Belle, Jefferson Co., Texas.
18.2 (17.8-18.5) _10_, San Antonio, Bexar Co., Texas.
18.2 (18.0-18.5) _5_, 2 mi. W Minaca, Chihuahua.
18.0 (17.6-19.0) _22_, 6 mi. SW San Geronimo, Coahuila.
18.2 (18.1-18.3) _3_, Ciudad Obregon, Sonora.
18.1 (17.4-18.5) _5_, vic. (see p. 649) Durango, Durango.
18.1 (17.5-18.5) _9_, Jaumave, Tamaulipas.
18.2 (17.7-18.9) _19_, 15 mi. N Rosario Chele, Sinaloa.
17.9 (17.4-18.3) _27_, vic. (see p. 655) Altamira, Tamaulipas.
18.3 (17.9-18.7) _9_, Valparaiso, Zacatecas.
18.1 (18.1-18.2) _4_, Ciudad del Maiz, San Luis Potosi.
18.6 (18.3-18.9) _8_, Tepic, Nayarit.
18.0 (17.7-18.4) _18_, 4 mi. N, 5 mi. W Leon, Guanajuato.
18.1 (17.5-18.9) _28_, 6 mi. E Queretaro, Queretaro.
17.7 (17.1-18.1) _17_, 1 mi. SSE Ameca, Jalisco.
17.3 (16.8-17.9) _10_, 2 mi. SSE Autlan, Jalisco.
18.0 (17.5-18.6) _10_, 1 mi. S, 11 mi. W Zamora, Michoacan.
17.6 (17.4-18.2) _8_, Colima, Colima.

_Baiomys musculus_

20.2 (19.9-20.3) _6_, vic. (see p. 622) Ameca, Jalisco.
20.2 (19.9-20.3) _6_, 2 mi. SSE Autlan, Jalisco.
19.6 (19.2-20.1) _6_, Jalapa, Veracruz.
20.3 (19.7-20.9) _9_, Colima, Colima.
19.5 (19.0-20.0) _10_, Cerro Gordo, Veracruz.
19.8 (19.4-20.3) _6_, 6 mi. S Izucar de Matemores, Puebla.
20.0 (18.8-20.5) _7_, Teotitlan, Oaxaca.
20.1 (19.7-20.7) _7_, 1 km. NW Chapa, Guerrero.
19.9 (19.4-20.4) _8_, 5 mi. ESE Tecpan, Guerrero.
19.5 (19.1-20.1) _22_, 3 mi. ESE Oaxaca, Oaxaca.
19.5 (19.1-19.9) _11_, Valley of Comitan, Chiapas.
18.9 (18.2-20.1) _17_, Tehuantepec, Oaxaca.
18.9 (18.4-19.7) _15_, 6 mi. NW Tonala, Chiapas.
19.1 (18.8-20.4) _10_, 1 mi. S Rabinal, Guatemala.
19.7 (18.8-20.4) _10_, Lake Amatitlan, Guatemala.
19.2 (18.4-19.8) _26_, vic. (see p. 625) San Salvador, El Salvador.
19.3 (18.9-19.9) _24_, 8 mi. S Condega, Esteli, Nicaragua.]

CONCLUSIONS

1. Two Recent species, each polytypic with eight subspecies, and five fossil species are recognized.

2. The phyletic trends in the genus _Baiomys_ have been from an ancestral stock that possessed relatively brachydont teeth having raised cingular ridges and orthodont to prooedont incisors, to species having hypsodont teeth with reduced cingular ridges and retrodont incisors.

3. Reduction of cingular ridges in pygmy mice is associated with an existence in open grassland (more xeric than mesic), whereas, the presence of cingular ridges is associated with an existence in a savannah habitat (more mesic than xeric).

4. Shifts of geographical range of populations of pygmy mice at and near the periphery of their geographic range may account for the differentiation of the extinct species.

5. The two living species, _B. musculus_ and _B. taylori_, are seemingly derived from a common ancestor that in morphological structure was intermediate between _B. minimus_ and _B. musculus_.

6. The living species of pygmy mice resulted from a geographic separation, perhaps occurring in the Iowan glacial period (See De Terra, 1949:51) in the transverse volcanic zone of central Mexico.

7. The two species are now sympatric in west central Mexico, where morphological characters (size and shape of body and length of skull) differ most. Where the two species are allopatric, these same morphological characters differ least.

8. This is a documented instance of character displacement in mammals.

9. On the basis of internal morphological characters studied (auditory ossicles, hyoid apparatus, and baculum), _Baiomys_ seems to be more closely related to a South American hesperomine, perhaps _Calomys_, than to any North American cricetine.

10. Pygmy mice were more widely distributed in the past than they are at present. Part of the ancestral stock of the pygmy mice may have emigrated from North America into South America in a brief period in the Pliocene; if so, it is easy to understand why certain South American hesperomines resemble _Baiomys_.

11. The combination of morphological and behavioral characters in the living pygmy mice warrants generic status for them. If _Baiomys_ were treated as a subgenus of the genus _Peromyscus_, there would be adequate justification for including in the genus _Peromyscus_ a number of other genera, some of them occurring in South America. Such lumping of genera would reduce our understanding of the natural relationships among this group of cricetine rodents.

LITERATURE CITED

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BOOTH, E. S.

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