taylori_ to the south and west, _subater_ is compared only with
_taylori_. Young adults of both subspecies in unworn pelage show best
the colors that differentiate the two subspecies. Old adults of
_subater_ in worn pelage appear grayish, resembling _taylori_, and at
that age, only certain cranial characters are of taxonomic use.
Cranially, _subater_ differs from _taylori_ in: presphenoid not shaped
like an hour-glass; parapterygoid processes thicker medially;
interparietal diamond-shaped instead of elongated and compressed. Skull
slightly larger in most measurements.
[Illustration: PLATE 1
Photographs of skulls in dorsal view of _Baiomys_. x 2.
_a._ _B. m. brunneus_, [F] ad., 10834, AMNH, Jalapa, Veracruz.
_b._ _B. m. grisescens_, [F] ad., 257080, USNM, Comayabuela, Honduras.
_c._ _B. m. handleyi_, [F] ad., 275597, USNM, Sacapulas, Guatemala.
_d._ _B. m. infernatis_, [F] ad., 91499, MZUM, Teotitlan, Oaxaca.
_e._ _B. m. musculus_, [F] ad., 45462, USNM, Colima, Colima.
_f._ _B. m. nigrescens_, [M] ad., 76834, USNM, Comitan, Chiapas.
_g._ _B. m. pallidus_, [F] ad., 4802, Texas A & M, Axochiapan, Morelos.
_h._ _B. m. pullus_, [F] ad., 71608, KU, 8 mi. S Condega, Nicaragua.
_i._ _B. t. allex_, [F] ad., 45453, USNM, Colima, Colima.]
[Illustration: PLATE 2
Photographs of skulls (_a-g_) in dorsal view of _Baiomys_. x 2.
_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.]
[Illustration: PLATE 3
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.]
[Illustration: PLATE 4
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.