Chapter I: Part 1

UNIVERSITY OF KANSAS PUBLICATIONS

MUSEUM OF NATURAL HISTORY

Volume 9, No. 23, pp. 579-670, 4 pls., 12 figs. in text

June 16, 1960

Speciation and Evolution of the
Pygmy Mice, Genus Baiomys

BY

ROBERT L. PACKARD

UNIVERSITY OF KANSAS
LAWRENCE
1960

UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman, Henry S. Fitch,

Robert W. Wilson

Volume 9, No. 23, pp. 579-670, 4 pls., 12 figs. in text
Published June 16, 1960

UNIVERSITY OF KANSAS
Lawrence, Kansas

PRINTED IN
THE STATE PRINTING PLANT
TOPEKA, KANSAS

1960

28-3030

Speciation and Evolution of the Pygmy Mice, Genus Baiomys

BY

ROBERT L. PACKARD

CONTENTS

PAGE
Introduction 583
Materials, Methods and Acknowledgments 584
Paleontology of the Genus 587
_Baiomys sawrockensis_ 588
_Baiomys rexroadi_ 589
_Baiomys kolbi_ 590
_Baiomys brachygnathus_ 590
_Baiomys minimus_ 591
Phyletic trends 592
Non-Geographic Variation 595
Variation with age 595
Secondary sexual variation 597
Individual variation 597
Pelage and molts 598
Taxonomic Characters and Relationships 600
External parts 600
Pelage 600
Skull 600
Teeth 601
Hyoid apparatus 601
Baculum 603
Auditory ossicles 605
Genus Baiomys 607
Systematic Accounts of Species and Subspecies 608
_Baiomys musculus_ 608
_Baiomys musculus brunneus_ 612
_Baiomys musculus grisescens_ 614
_Baiomys musculus handleyi_ 617
_Baiomys musculus infernatis_ 618
_Baiomys musculus musculus_ 620
_Baiomys musculus nigrescens_ 623
_Baiomys musculus pallidus_ 625
_Baiomys musculus pullus_ 628
_Baiomys taylori_ 630
_Baiomys taylori allex_ 633
_Baiomys taylori analogous_ 637
_Baiomys taylori ater_ 640
_Baiomys taylori canutus_ 643
_Baiomys taylori fuliginatus_ 645
_Baiomys taylori paulus_ 647
_Baiomys taylori subater_ 650
_Baiomys taylori taylori_ 651
Evolution and Speciation 655
Formation of the Recent Species 658
Areas of present differentiation 661
Zoogeographic position 661
Conclusions 664
Literature Cited 665

INTRODUCTION

Pygmy mice (_Genus Baiomys_) are the smallest cricetine rodents in North America. They occur from Nicaragua in Central America into the southwestern United States. The principal part of the geographic range of the pygmy mice lies in the Republic of Mexico. They are notably common in central Mexico, but are only locally common to the north and to the south, and then only in certain seasons.

Pygmy mice were first brought to the attention of biologists in 1887 when Oldfield Thomas described a diminutive species of cricetine rodent, _Hesperomys_ (_Vesperimus_) _taylori_. The description was based on a specimen obtained by William Taylor from San Diego, Duval County, Texas. C. Hart Merriam (1892:70) described _Sitomys musculus_ on the basis of specimens from Colima [City of], Colima, Mexico. Merriam (_loc. cit._) mentioned that the two kinds of mice, _Hesperomys taylori_ and _Sitomys musculus_, "in general appearance look almost precisely like the common house mouse (_Mus musculus_) but are still smaller and have shorter tails." He placed the two species in the genus _Sitomys_. Frederick W. True in 1894 regarded them as composing a distinct subgenus of _Sitomys, Baiomys_. According to True (1894:758), _S. taylori_ and _S. musculus_ possessed a different combination of characters (ascending ramus of mandible short and erect, condyle terminal, coronoid process well-developed, uncinate, and near the condyle, size small, tail short, plantar tubercles six, soles hairy) than either _Vesperimus_, or _Onychomys_ (which had been considered as a subgenus of _Hesperomys_ until 1889). In 1907, E. A. Mearns accorded _Baiomys_ generic rank. Osgood (1909:252) treated _Baiomys_ us a subgenus of _Peromyscus_, whereas, Miller, in 1912, regarded _Baiomys_ as a distinct genus. Most recent students of North American mammals have followed Miller, but usually with reservations. Ellerman (1941:402) emphasized that the taxonomic position of the genus was uncertain, and wrote that _Baiomys_ "... seems to be considerably distinct from _Peromyscus_, and may perhaps be a northern representative of _Hesperomys_ or one of the small South American genera."

Only two comprehensive analyses of geographic variation and interspecific taxonomic relationships have been made; the first was by Osgood (1909) who had fewer than a fourth of the specimens of _Baiomys_ available to me; the second was by Hooper (1952a:90-97) who contributed importantly to understanding the relationships of the two living species in central Mexico. No attempts heretofore have been made to correlate and understand the relationships of the five fossil species to one another and to the living species assigned to the genus.

Six objectives of the following report are to: (1) list characters taxonomically useful in recognizing species and subspecies; (2) record amount of variation within and between populations; (3) correlate observed variations with known biological principles; (4) show geographic ranges of the two living species; (5) indicate relationships between fossil and living species of the genus; and (6) clarify the systematic position of the genus.

MATERIALS, METHODS AND ACKNOWLEDGMENTS

This report is based on the study of approximately 3,520 museum study skins, skulls, complete skeletons, and entire animals preserved in liquid. Most specimens examined were accompanied by an attached label bearing data on locality and date of capture, name of collector, external measurements, and sex. In addition, 49 fossil specimens referable to _Baiomys_ were studied. Nearly two-thirds of the specimens were assembled at the University of Kansas Museum of Natural History; the remainder were examined in other institutions.

Specimens studied were grouped by geographic origin, sex, age, and season of capture. Individual variation was then measured in several of the larger samples of each living species and in measurable fossil material. External measurements used were those recorded by the collectors on the labels attached to the skins. Twenty cranial measurements employed in the past in the study of _Baiomys_ and closely related cricetine rodents were statistically analyzed. The coefficient of variation was calculated for each of the 20 measurements in order to determine which varied least. In general, measurements having the least coefficient of variation were used in comparing samples from different geographic areas. Figure 1 shows the points between which measurements were taken.

_Occipitonasal length._--From anteriormost projection of nasal bones to posteriormost projection of supraoccipital bone. _A_ to _A'_

_Zygomatic breadth._--Greatest distance across zygomatic arches of cranium at right angles to long axis of skull. _B_ to _B'_

_Postpalatal length._--From posterior margin of hard palate to anterior margin of foramen magnum. _C_ to _C'_

_Least interorbital breadth._--Least distance across top of skull between orbits. _D_ to _D'_

_Length of incisive foramina._--From anteriormost point to posteriormost point of incisive foramina. _E_ to _E'_

_Length of rostrum._--The distance in a straight line from the notch that lies lateral to the lacrimal to the tip of the nasal on the same side. _F_ to _F'_

_Breadth of braincase._--Greatest distance across braincase, taken at right angles to long axis of skull. _G_ to _G'_

_Depth of cranium._--The distance from the dorsalmost part of the braincase to a flat plane touching tips of incisors and ventral border of each auditory bulla. A glass slide one millimeter thick was placed on the ventral side of the skull. One jaw of the caliper was on the lower surface of the slide and the other jaw on the dorsalmost part of the braincase. The depth of the slide was subtracted from the total reading. _H_ to _H'_

_Alveolar length of maxillary tooth-row._--From anterior border of alveolus of M1 to posterior alveolus of M3. _I_ to _I'_

Capitalized color-terms refer to Ridgway (1912). Color terms without initial letters capitalized do not refer to any one standard.

The names of the cusps and ridges of the teeth (see Figure 2) are those suggested by Wood and Wilson (1936:389-390). Terminology of the enamel grooves and folds is that of Hershkovitz (1944:17) and Hooper (1952b:20-21).

Because secondary sexual variation was not significant (see page 597), both males and females of like age and pelage were used in comparisons of samples designed to reveal geographic variation.

The species are arranged from less to more progressive; the subspecies are arranged alphabetically.

In the synonymy of each subspecies, the plan has been to cite: (1) the name first proposed; (2) the first usage of the name combination employed by me; (3) all other name combinations in chronological order that have been applied to the subspecies concerned.

The localities of specimens examined are listed by country from north to south. Within a country, the listing is by state, beginning with the northwesternmost state and proceeding by tiers (west to east) to the southeasternmost state. Within a state of the United States, the listing is by counties in the same geographic order as described for states. Within any county in the United States, within any state in Mexico, and within any country in Central America, the listing of localities is from north to south. When more than one locality is on the same line of latitude, the westernmost locality is listed first. Marginal localities for each subspecies are listed in a paragraph at the end of each account. Each marginal locality is mapped by means of a circle. The circles are listed in clockwise order, beginning with the northernmost. When more than one of these localities lies on the same line of latitude, the westernmost is cited first. Localities not represented on the distribution maps, so as to avoid undue crowding of symbols, are italicized in the lists of specimens examined.

A. _B. taylori analogous_, subadult, female, No. 28102 KU, 4 km.
ENE Tlalmanalco, 2290 meters, Estado de Mexico. Right, upper
molars.

B. _B. musculus musculus_, subadult, male, No. 45456 USNM, Colima,
Colima, Mexico. Left, upper molars.

A'. _B. taylori analogous_, subadult, female, No. 28102 KU 4 km.
ENE Tlalmanalco, 2290 meters, Estado de Mexico. Left, lower
molars.

B'. _B. musculus musculus_, subadult, male, No. 45456 USNM, Colima,
Colima, Mexico. Right, lower molars.]

The largest single collection of pygmy mice is in the University of Kansas Museum of Natural History, and, unless otherwise indicated, specimens cited in the taxonomic accounts beyond are there.

I am indebted to the following named institutions and persons for making specimens available for study:

American Museum of Natural History, G. G. Goodwin and R. G. VanGelder.

Carnegie Museum, J. K. Doutt.

California Academy of Sciences, Robert T. Orr.

Chicago Natural History Museum, Phillip H. Hershkovitz.

Cleveland Museum of Natural History (Collection now a part of Museum of
Zoology, University of Michigan, W. H. Burt, E. T. Hooper).

Louisiana State University, Museum of Natural History, George H. Lowery,
Jr.

Los Angeles County Museum, Charles A. McLaughlin.

United States National Museum (Biological Survey Collections), David A.
Johnson, and Viola S. Schantz.

United States National Museum, Division of Vertebrate Paleontology,
C. Lewis Gazin.

University of Arizona, E. L. Cockrum, and G. VR. Bradshaw.

University of California, Museum of Vertebrate Zoology, Seth B. Benson,
and W. Z. Lidicker.

University of Illinois, Museum of Natural History, Donald F.
Hoffmeister.

University of Michigan, Museum of Zoology, W. H. Burt, E. T. Hooper, and
Claude W. Hibbard.

University of New Mexico, James S. Findley.

University of Texas, Frank W. Blair.

Texas A & M, Cooperative Wildlife Research Collection, W. B. Davis.

The Museum, Michigan State University, Rollin H. Baker.

University of Florida Collections, James N. Layne.

I am especially grateful to Professor E. Raymond Hall who guided me in my study and gave critical assistance with the manuscript. Additional appreciated suggestions were made by Professors A. Byron Leonard, Robert W. Wilson, Henry S. Fitch, Ronald L. McGregor, and fellow graduate students. For the illustrations, I am indebted to Mrs. Lorna Cordonnier, Miss Lucy Remple and Mrs. Connie Spitz. Mr. B. J. Wilks of the University of Texas, Department of Zoology, provided a number of living pygmy mice for study in captivity. Mr. J. Raymond Alcorn and his son, Albert, collected a large share of specimens of pygmy mice now in the University of Kansas, Museum of Natural History. My wife, Patricia, aided me in secretarial work and typing of the manuscript.

For financial assistance, I am indebted to the National Science Foundation when I was a Research Assistant, to the Sigma Xi-RESA Research Fund for a Grant-in-Aid, and to the Kansas University Endowment Association through its A. Henley Aid Fund, and the Watkins Fund for out-of-state field work by the Museum of Natural History.

PALEONTOLOGY OF THE GENUS

Five fossil species, all extinct, have been assigned to the genus and range in time from early late Pliocene (Saw Rock Canyon fauna of Hibbard, 1953:408) to Mid-Pleistocene (see Hibbard, 1958:25, who assigns the Curtis Ranch fauna to late Kansan or early Yarmouth).

I examined all known fossil material and compared it with Recent material. When the antiquity of the genus is considered, the degree of difference between the oldest fossil species and the two living species is much less than might be expected.

=Baiomys sawrockensis= Hibbard

_Baiomys sawrockensis_ Hibbard, Papers Mich. Acad. Sci., Arts and
Letters, 38:402, April 27, 1953.

_Type._--No. 27506, Univ. Michigan; left mandibular ramus bearing m1-m3 and incisor; Saw Rock Canyon, early late Pliocene, XI member of the Rexroad formation, sec. 36, T. 34 S, R. 31 W, Seward County, Kansas (University of Kansas, Locality 6).

_Referred material._--Univ. Michigan, Nos. 25781, 27503-27505, 28159-28165, 29708-29715, 31015.

_Diagnosis._--Ramus of medium size to small for the genus; lower incisor broad, moderately recurved; diastemal region broad; anterior median fold between anterior labial conulid and anterior lingual conulid of m1 deep; primary first fold between anteroconulid and protoconid of m2 deep; cingular ridge (ectolophid) at entrance to posteroexternal reentrant valley (major fold, see Figure 2) between protoconid and hypoconid of m1 and m2; average and extreme measurements of lower molar row of eight specimens are, 2.65 (2.5-2.7).

_Comparisons._--For comparisons with _B. brachygnathus_, see account of that species. From _B. rexroadi_, _B. sawrockensis_ differs in: anterior median fold of m1 deeper; incisor narrower; diastemal region broader; coronoid process broader and better developed; cingular ridges (ectolophids and mesolophids) more pronounced in their development; incisors less prooedont, more retrodont.

From _B. kolbi_, _B. sawrockensis_ differs in: crowns of molars narrower; incisors less prooedont; cingular ridges (ectolophids and mesolophids) of m1 and m2 more pronounced in their development.

From _B. minimus_, _B. sawrockensis_ differs in: incisor less procumbent; masseteric ridge extending farther anteriorly; anterior cingulum of m2 slightly larger.

From _B. musculus_, _B. sawrockensis_ differs in: over-all size of jaw and molar row less; diastema more acutely curved; incisors shorter; anterior median fold of m1 slightly deeper.

From _B. taylori_, _B. sawrockensis_ differs in: m1 and m2 smaller; cingular ridges in m1 and m2 more pronounced; anterolingual conulid farther forward; incisors shorter, more prooedont; molar teeth depressed, less hypsodont; diastemal region broader, more acutely curved; masseteric ridge not extending so far anteriorly.

_Remarks._--_B. sawrockensis_ is the oldest known pygmy mouse. The extreme development of the anterior median fold between the anterolingual conulid and the anterolabial conulid is regarded as a primitive feature in the pygmy mice. In this character, the Recent species can be traced back in time through _B. minimus_ to _B. sawrockensis_. _B. sawrockensis_ resembles _Calomys laucha_ of South America in general conformation of jaw and tooth structure. The molars of _sawrockensis_ are smaller than those of _C. laucha_, and the anterolingual conulid of _sawrockensis_ is farther forward.

=Baiomys rexroadi= Hibbard

_Baiomys rexroadi_ Hibbard, Amer. Midland Nat., 26:351, September,
1941; Hibbard, Contrib. Mus. Paleo., Univ. Michigan, 8(2):145,
June 29, 1950 (part); Hibbard, Papers Mich. Acad. Sci., Arts
and Letters, 38:403, April 27, 1953.

_Type._--No. 4670, Univ. Kansas; left mandibular ramus bearing m1-m3, and incisor; Rexroad fauna, Locality no. 2, Upper Pliocene, Meade County, Kansas.

_Referred material._--Univ. of Michigan Nos. 24840, 24851, 27493, 27496, 27501, 28862-28867.

_Diagnosis._--Ramus medium in size for the genus; incisors small, prooedont; anterior median fold of m1 slight; cingulum of all molars poorly developed; average and external measurements of lower molar row of seven specimens are, 2.7 (2.6-3.0).

_Comparisons._--For comparisons with _B. sawrockensis_ and _B. minimus_, see accounts of those species. From _B. kolbi_, _B. rexroadi_ differs in: over-all size of mandibular ramus, incisors, and molars smaller; anterior median fold of m1 present, though poorly developed.

From _B. brachygnathus_, _B. rexroadi_ differs in: over-all size of mandibular ramus smaller; m3 larger; posterior cusps (hypoconid and entoconid) elongated; diastema shorter, less acutely recurved; incisors less prooedont; cingular ridges of m1 and m2 less well-developed.

From _B. musculus_, _B. rexroadi_ differs in: over-all size of mandibular ramus less; cingular ridges of m1 and m2 less well-developed; incisors smaller, more prooedont; molars less depressed.

From _B. taylori_, _B. rexroadi_ differs in: m3 more triangular, posterior part narrower; mental foramen closer to anterior root of m1; masseteric ridge closer to alveolus of m1; incisor shorter, more prooedont; molars more depressed.

_Remarks._--Two maxillary tooth-rows and associated parts were studied. On one of these specimens, the M2 has a well-developed mesostyle; the anterior median fold of M1 is also well-developed. The other specimen possesses a low cingular ridge (enteroloph) between the protocone and the hypocone, a reduced cingular ridge (mesoloph) between the paracone and metacone of M1. On the second molar, M2, a mesostyle joins with the mesoloph somewhat in the fashion indicated by Hooper (1957:9, encircled number 2).

=Baiomys kolbi= Hibbard

_Baiomys kolbi_ Hibbard, Trans. Kansas Acad. Sci., 55:201, June 18,
1952; Hibbard, Papers Mich. Acad. Sci., Arts and Letters, 38:403,
April 27, 1953.

_Type._--No. 24846, Univ. Michigan; right mandibular ramus bearing m1-m3 and incisor; Fox Canyon, upper Pliocene, Rexroad formation, Rexroad fauna, Univ. Michigan Locality K1-47, sec. 35, T. 34 S, R. 30 W, XI Ranch, Meade County, Kansas.

_Referred material._--Univ. Michigan Nos. 24845-24848, 27494, 27497, 27499, 28566, 28861, 28878, 28880-28882, 28884, 28886.

_Diagnosis._--Ramus of medium size to large for the genus; lower incisor short, narrow transversely, prooedont; anterior median fold of m1 reduced or absent; cingular ridges of m1 and m2 moderately well-developed; m3 large relative to m1 and m2; average and extreme measurements of lower molars of seven specimens are, 3.0 (3.0-3.1).

_Comparisons._--For comparisons with _B. sawrockensis_ and _B. rexroadi_, see accounts of those species. From _B. brachygnathus_, _B. kolbi_ differs in: molar row longer; m3 and jaw larger; diastema longer; masseteric ridge not so far forward; molars more depressed.

From _B. minimus_, _B. kolbi_ differs in: molar row longer; m3 larger; jaw larger; diastema not so acutely curved; incisor shorter, narrower transversely, more prooedont.

From _B. musculus_, _B. kolbi_ differs in: anterior median fold of m1 slightly developed or absent, instead of well-developed; m3 larger (not reduced), external reentrant valley broad and extending farther across crown of tooth; incisor smaller, and more prooedont; cingular ridges of m1 and m2 less well-developed.

From _B. taylori_, _B. kolbi_ differs in: molars larger, more depressed; incisor shorter, more prooedont; m3 smaller relative to m1 and m2; external reentrant valley of m3 broad, extending farther across crown of tooth.

_Remarks._--The slight development or absence of the anterior median fold in _kolbi_ suggests that it was specialized. The anterior median fold is well-developed in all species of _Baiomys_ save _B. brachygnathus_ and _B. taylori_, in which the fold is only slightly developed or absent. _B. kolbi_ may have paralleled _B. taylori_ in specialization for a diet of grasses and for a life in open country.

=Baiomys brachygnathus= (Gidley)

_Peromyscus brachygnathus_ Gidley, U. S. Geol. Surv. Prof. Papers,
131:124, March 15, 1922.

_Baiomys brachygnathus_, Hibbard, Amer. Midland Nat., 26:352,
September, 1941.

_P. [eromyscus] brachygnathus_, Wilson, Carnegie Inst. Washington
Publ., 473:33, May 21, 1936.

_Type._--No. 10501, U. S. Nat. Mus.; right mandibular ramus bearing m1-m3, and incisor; 2 mi. NE Curtis Ranch house, near a line between sec. 28 and 29, T. 18 S, R. 21 E, Mid-Pleistocene (Hibbard, 1958:25), Cochise County, Arizona.

_Referred material._--None.

_Diagnosis._--Ramus small for the genus; m3 reduced; jaw reduced anteroposteriorly; incisor short, slender, prooedont; cingular ridges well-developed, posterior ectolophid continuous from protoconid to hypoconid in m1 and m2; diastema short; length of molar row 2.8 mm.

_Comparisons._--For comparisons with _B. rexroadi_ and _B. kolbi_, see accounts of those species. From _B. minimus_, _B. brachygnathus_ differs in: jaw not so slender anteriorly; masseteric ridge not so far anterior; cheek-teeth slightly broader, less depressed, therefore, more hypsodont; incisor shorter, more prooedont.

From _B. sawrockensis_, _B. brachygnathus_ differs in: molar row slightly longer; teeth slightly less depressed; masseteric ridge extends farther anteriorly; incisors more prooedont.

From _B. musculus_, _B. brachygnathus_ differs in: jaw smaller; molar row slightly shorter; molars less depressed; incisors slender, shorter, narrower, and more prooedont.

From _B. taylori_, _B. brachygnathus_ differs in: incisor more slender, shorter, more prooedont; diastema shorter.

_Remarks._--The molar teeth of _B. brachygnathus_, although worn, resemble those of _B. taylori_ more than those of any known fossil species. Gidley (1922:124) stated that the absence of the divided anterior lobe of the first molar (anterior median fold) in _brachygnathus_ was one of the chief characters separating _brachygnathus_ from _taylori_. In _taylori_, the anterior median fold characteristically is only slightly developed, and in some specimens is absent. _B. brachygnathus_ differs from _taylori_ chiefly in prooedont incisors, which feature seems to preclude _brachygnathus_ being ancestral to _taylori_. _B. brachygnathus_ may have been a specialized divergence from _B. minimus_.

=Baiomys minimus= (Gidley)

_Peromyscus minimus_ Gidley, U. S. Geol. Surv. Prof. Papers,
131:124, March 15, 1922.

_Baiomys minimus_, Hibbard, Amer. Midland Nat., 26:352, September,
1941; Gazin, Prof. U. S. Nat. Mus., 92(3155):488, 1942.

_P. [eromyscus] minimus_, Wilson, Carnegie Inst. Washington Publ.,
473:33, May 21, 1936.

_Type._--No. 10500, U. S. Nat. Mus.; left mandibular ramus bearing m1-m3 and incisor; 2 mi. S Benson, sec. 22, T. 17 S, R. 20 E, Late Pliocene (Blancan, Gazin, 1942:482), Cochise County, Arizona.

_Referred material._--None.

_Diagnosis._--Ramus small for the genus; molar teeth depressed; cingular ridges (ectolophids) of m1 and m2 well-developed; anterior median fold present (appearing larger owing to chip of enamel missing); external reentrant fold of m3 progresses half way across crown of tooth; diastema short; incisor moderately large, recurved; length of molar row, 2.6 mm.

_Comparisons._--For comparisons with _B. brachygnathus_, _B. kolbi_, and _B. sawrockensis_, see accounts of those species. From _B. rexroadi_, _B. minimus_ differs in: anterior median fold deeper; incisor longer, more recurved, less prooedont; molars slightly more depressed (though worn).

From _B. musculus_, _B. minimus_ differs in: over-all size of jaw and molars smaller; incisors shorter; masseteric ridge more depressed.

From _B. taylori_, _B. minimus_ differs in: anterior median fold slightly deeper; molar teeth more depressed; cingular ridges on m1 and m2 better developed; masseteric ridge more depressed.

_Remarks._--Gidley (1922:124) stated that _B. minimus_ differed considerably from _B. taylori_ in that the coronoid portion of the ascending ramus diverges at a wider angle from the alveolar part of the jaw. Study of large samples of lower jaws of _B. taylori_ reveals considerable individual variation in the angle formed between the coronoid part of the jaw and the alveolar part.

_B. minimus_, except for its small size, is like _B. musculus_ and is considered to be ancestral to that species.

PHYLETIC TRENDS

It seems that the important trends in phyletic development in the pygmy mice have been from an ancestral stock (see Figure 3) that possessed relatively brachydont teeth having raised cingular ridges (ectolophids and mesolophids) and relatively short orthodont to prooedont incisors, to species having teeth more hypsodont on which cingular ridges were reduced, stylids were isolated or completely absent, and incisors were longer and more recurved or retrodont. _Baiomys sawrockensis_, or an unknown stock resembling it, might have been ancestral to the other known species. Of the four remaining fossil species, _B. kolbi_ seems least likely to have been ancestral to the two living species, owing to its prooedont incisors, reduction of cingular ridges, loss of an anterior median fold in m1, and long mandibular tooth-row. _B. kolbi_ may have been an early, specialized derivation from the ancestral stock. From his knowledge of the habitats of _B. musculus_, the larger species, and _B. taylori_, the smaller species, Hibbard (1952:203) suggests that _B. kolbi_, a large species, might have inhabited lowlands, and _B. rexroadi_, a small species, highlands. I have no evidence to dispute this suggestion except that _B. musculus_ has more prominent cingular ridges (or at least vestiges of this lophid condition) than either _B. kolbi_ or _B. rexroadi_. _B. musculus_ (see page 610) is less of an open grassland inhabitant than is _B. taylori_. Therefore, both _B. kolbi_ and _B. rexroadi_, because of their poorly developed cingular ridges, might be expected to have lived in a relatively open grassland habitat.

The relationship of _B. rexroadi_ to fossil species other than _B. kolbi_ is not clear. Superficially, the former resembles _B. taylori_, but, owing to the specialized development of the molars of _rexroadi_, it could hardly have been ancestral to either of the living species. The resemblance of _B. rexroadi_ to _B. taylori_ may result from each having occupied the same ecological niche in different periods. The incisors of _B. rexroadi_, however, are much shorter than those of _B. taylori_ and suggest somewhat different food habits.

_B. minimus_ seemingly is more closely related to _B. sawrockensis_ and _B. musculus_ than to the other described species. The development of the cingular ridges leads one to suspect that _B. minimus_ was the ancestor of _B. musculus_. _B. minimus_ may have been derived from a _sawrockensis_-like stock and probably gave rise to _B. musculus_.

Hershkovitz (1955:643-644) suggests that "... primitive brachydont, buno-mesolophodont cricetines have survived ... in forested parts of the range," whereas "... the progressive branch of cricetines with mesoloph absent or vestigal, has become increasingly specialized for life in open country and a diet of grasses." Species of the genus _Baiomys_ can be divided into two morphological groups. One group, composed of _B. sawrockensis_, _B. minimus_, and _B. musculus_, includes those species, the teeth of which were relatively brachydont and had prominently developed cingular ridges (ectolophids or mesolophids) or, at least, showed some development of these ridges. _B. sawrockensis_ probably lived in semi-wooded to shrubby habitats. According to Hibbard (1953:409), "The Saw Rock Canyon fauna lived in that area at a time when conditions were comparable to the conditions at the time the Rexroad fauna lived." The conditions in which the Rexroad fauna lived are discussed by Hibbard (1941:95). Presumably, there were at least some well-wooded situations, and the climate was warm. _B. sawrockensis_ probably inhabited denser vegetation than did _B. minimus_ or than does _B. musculus_. The teeth of the second group (_B. kolbi_, _B. rexroadi_, _B. brachygnathus_, and _B. taylori_) lack cingular ridges or have them much reduced and have more hypsodont molars. The three fossil species probably inhabited relatively open grassland. This assumption is based largely on the known habitat of _B. taylori_ (see page 632).

The suggested grouping, based on supposed similarities in niches inhabited by the extinct species, does not necessarily indicate degree of relationship. _B. taylori_ probably was not derived from an ancestor like _B. rexroadi_ or _B. kolbi_, although, in certain characters, the three species resemble one another. _B. kolbi_ and _B. rexroadi_ were already specialized in Blancan times, probably for living on grassland. _B. taylori_ shows only a slight advance in specialization of molar structures compared to either of the aforementioned species but is slightly smaller and does have longer and more recurved incisors. If only morphological criteria of lower jaws were considered, without recourse to other data derived from the study of many samples of populations of the living species, time alone might account for the differences among _B. taylori_, _B. rexroadi_, and _B. kolbi_. The available evidence (see page 658) suggests, however, that _B. taylori_ was derived from the _B. sawrockensis_-_B. minimus_-_B. musculus_ line.

_Baiomys_ seems to have undergone little basic evolutionary and morphological change since Late Pliocene time. According to Simpson (1945:207), hesperomine rodents as a group have undergone little basic evolution, and "The rapid evolution of new genera was more a matter of segregation of characters in a group with a great variation than of the origin of significantly new characters." Perhaps, the living southern pygmy mouse retains many basic characteristics of one of the early North American cricetine-like stocks that emigrated to South America near the end of the Pliocene epoch. There is much to suggest close relationship of the pygmy mice to certain species of South American hesperomine rodents of the genus _Calomys_.

NON-GEOGRAPHIC VARIATION

Non-geographic variation in pygmy mice (variation in a single population resulting from age, individual, seasonal, and secondary sexual differences) has been but little studied in the past. Mearns (1907:381) figured progressive stages of wear on the teeth of _B. taylori_; Osgood (1909:252) and Blair (1941:380) referred to changes in dentition, weights, and pelages.

The largest samples available for this study were 47 _B. taylori_ from the vicinity of Altamira (6 mi. N, 6 mi. W; 5 mi. N, 5 mi. W; 1 mi. S), Tamaulipas, and 44 _B. musculus_ from El Salvador (1 mi. S Los Planes, and 1 mi. NW San Salvador--two localities 3 miles apart).

VARIATION WITH AGE

Specimens of both species were segregated into five categories: Juveniles, young, subadults, adults, and old adults. Juvenal and young pygmy mice are readily separable from the other three categories; subadults are less easily distinguished from adults. In order to obtain an accurate understanding of geographic variation in these mice, only adults should be used in making taxonomic comparisons.

_Juveniles._--Nestling mice yet unweaned; sutures in cranium incompletely closed; bony parts of skull fragile; M3 and m3 not erupted or only partly erupted and not protruding above margins of alveoli.

At birth, juveniles are pink, without pelage except for the mystacial vibrissae and a few hairs about the eye. Blair (_op. cit._:381) recorded changes with age in color of the skin of new-born and suckling pygmy mice. Data obtained by me from three litters born in captivity agree with his findings. Pygmy mice are weaned when 17 to 24 days old. At that time, the mice possess a fine, but not dense, dusky-gray fur.

_Young._--Weaned mice; cranium fragile; sutures between frontals and parietals, interparietal and parietals, basioccipital and basisphenoid, basisphenoid and presphenoid, premaxillaries and maxillaries widely open; M3 and m3 erupted beyond margins of their alveoli (molars erupt from anterior to posterior; M3 and m3, therefore, are last to erupt); in some specimens, molars slightly worn; pelage still dusky and relatively fine and sparse.

_Subadults._--Sutures between bones of skull less widely open than in young; epiphyses of long bones incompletely coalesced to shaft; relative to length of skull, braincase higher and rostrum shorter than in adults; all cusps worn, but dentine not occlusally confluent; primary first and second folds of third upper molars present; primary first fold and major fold of lower molars visible; pelage a subtle mixture of colors of young and adult, but resembling most that of adult; molts into postjuvenal pelage between 46 and 50 days.

_Adults._--Sutures of skull, and those between epiphyses and shaft of long bones obliterated except that, in some mice, sutures of skull persist between frontoparietal, and interparietal; cusps of molars so worn that dentine occlusally confluent; small island of enamel in third upper and lower molars of some specimens; relative to length of skull, cranium lower, rostrum longer, and interorbital region narrower than in subadult; cranium appears to be more flattened dorsoventrally; between subadult and adult stages, principal growth occurs in basioccipital, basisphenoid, frontals, and parietals; nasals grow less.

Although all bones of the skull grow in the subadult and early adult stages (see table 1), the above-named bones grow faster than others and thus cause the general flattening of the skull, typical of adults (similar to that reported by Hoffmeister, 1951:7). The body continues to lengthen, accounting for the increase in total length of the adult (see table 1). Hind foot, tail and ear, reach their maximum lengths by subadult stage. Adult pelage has been acquired, and the color is brighter than in either subadults or old adults.

_Old Adults._--Characterized principally by well-worn molars; only thin peripheral band of enamel along with slight evidence of any primary or secondary folds on any teeth remain; all bones of skull coalesced; epiphyses and shafts of long bones ankylosed; small bony protuberances on many skulls; pelage usually ragged, tips of the hairs being worn away; white flecking and spotting not common, but occurs in some adults.

TABLE 1.--Average and Extreme Measurements (in Millimeters) of
Skulls of Five Age-groups of Baiomys taylori from vic.
(see p. 595) Altamira, Tamaulipas, Mexico.

=============+===========+===========+===========+===========+===========
Age groups | Juvenile | Young | Subadult | Adult | Old adult
-------------+-----------+-----------+-----------+-----------+-----------
Number | | | | |
examined | 3 | 3 | 14 | 19 | 8
| | | | |
| | | | |
Total length | 77.0 | 92.6 | 97.6 | 99.9 | 101.6
| (74-79) | (89-96) | (91-103)| (93-105) | (98-107)
| | | | |
| | | | |
Length | 27.3 | 39.3 | 40.4 | 39.8 | 40.9
of tail | (24-29) | (37-41) | (36-43) | (35-45) | (38-45)
| | | | |
| | | | |
Length | 49.6 | 53.3 | 57.0 | 60.0 | 60.7
of body | (49-50) | (52-55) | (51-61) | (56-67) | (57-67)
| | | | |
| | | | |
Length of | 11.0 | 13.6 | 14.3 | 14.5 | 14.2
hind foot | (11) | (13-14) |(13.5-15.0)| (14-15) | (13-15)
| | | | |
| | | | |
Occipitonasal| 14.2 | 16.3 | 17.1 | 17.7 | 17.8
length |(13.6-15.2)|(15.8-16.9)|(16.7-17.6)|(17.2-18.3)|(17.6-18.1)
| | | | |
| | | | |
Zygomatic | 8.1 | 8.7 | 8.9 | 9.3 | 9.4
breadth | (7.8-8.6) | (8.6-8.8) | (8.6-9.3) | (9.0-9.6) | (9.1-9.6)
| | | | |
| | | | |
Interorbital | 3.4 | 3.4 | 3.4 | 3.6 | 3.5
breadth | (3.3-3.5) | (3.3-3.6) | (3.3-3.6) | (3.4-3.8) | (3.3-3.6)
| | | | |
| | | | |
Incisive | | | | |
foramina | 2.9 | 3.5 | 3.7 | 3.9 | 3.9
(length) | (2.8-2.9) | (3.4-3.6) | (3.6-3.9) | (3.6-4.1) | (3.5-4.0)
| | | | |
| | | | |
Depth | 5.9 | 6.5 | 6.5 | 6.7 | 6.8
of cranium | (5.6-6.2) | (6.3-6.8) | (6.2-6.8) | (6.4-7.0) | (6.5-7.1)
| | | | |
| | | | |
Alveolar | | | | |
length, | 2.7 | 2.9 | 2.9 | 3.0 | 3.0
upper molars | (2.5-2.8) | (2.9-3.0) | (2.8-3.1) | (2.9-3.2) | (3.0-3.1)
| | | | |
| | | | |
Postpalatal | 4.8 | 5.9 | 6.2 | 6.5 | 6.5
length | (4.5-5.3) | (5.8-6.0) | (5.8-6.6) | (6.2-7.2) | (6.3-6.7)
| | | | |
| | | | |
Breadth | 8.1 | 8.5 | 8.4 | 8.6 | 8.6
of braincase | (7.8-8.7) | (8.5) | (8.0-8.7) | (8.3-8.9) |(8.4-8.8)
-------------+-----------+-----------+-----------+-----------+-----------

SECONDARY SEXUAL VARIATION

The method employed by Dice and Leraas (1936:2) was used to measure the secondary sexual differences, if there were any, in each of several age classes. As pointed out by Hooper (1952b:11), individual variation in small samples can obscure secondary sexual differences. The samples of _B. taylori_ from the vicinity (see page 595) of Altamira, Tamaulipas, and the samples of _B. musculus_ from El Salvador (table 2) were large enough to prevent individual variation from obscuring sexual differences. Nevertheless, no significant secondary sexual differences were found in either _B. taylori_ or _B. musculus_ (see table 2). Therefore, the sexes have been considered together for purposes of geographic studies.

TABLE 2.--Analysis of Secondary Sexual Variation in Adult B. taylori
Vicinity of (see p. 595) Altamira, Tamaulipas, and Adult B.
musculus from El Salvador (see p. 595). (One Standard Deviation
on Either Side of the Mean is Given.)

==============+==========================+============================
| Baiomys taylori | Baiomys musculus
Character +------------+-------------+-------------+--------------
| 21 Males | 18 Females | 17 Males | 13 Females
--------------+------------+-------------+-------------+--------------
| | | |
Total length |98.4 +- 2.95 |100.5 +- 4.72 |112.04 +- 5.49|113.12 +- 4.23
| | | |
Length of tail|40.1 +- 2.31 | 40.3 +- 2.39 | 47.12 +- 2.95| 45.70 +- 2.92
| | | |
Length of body|57.83 +- 1.65| 60.10 +- 4.13| 66.67 +- 3.97| 67.75 +- 2.38
| | | |
Length of | | | |
hind foot |14.21 +- .53 | 14.44 +- .51 | 15.60 +- .49 | 15.38 +- .64
| | | |
Length of ear |10.00 +- .00 | 10.00 +- .00 | 11.80 +- .65 | 12.00 +- .41
| | | |
Occipitonasal | | | |
length |17.48 +- .40 | 17.47 +- .47 | 19.32 +- .35 | 19.04 +- .44
| | | |
Zygomatic | | | |
breadth | 9.17 +- .33 | 9.15 +- .30 | 9.84 +- .21 | 9.91 +- .28
| | | |
Least | | | |
interorbital | | | |
breadth | 3.53 +- .11 | 3.48 +- .11 | 3.88 +- .08 | 3.88 +- .12
| | | |
Postpalatal | | | |
length | 6.35 +- .19 | 6.38 +- .30 | 7.11 +- .15 | 6.95 +- .20
| | | |
Depth | | | |
of cranium | 6.65 +- .24 | 6.61 +- .17 | 7.10 +- .18 | 7.08 +- .18
| | | |
Incisive | | | |
foramina | | | |
(length) | 3.82 +- .15 | 3.81 +- .18 | 4.43 +- .11 | 4.35 +- .14
| | | |
Length | | | |
of rostrum | 5.87 +- .20 | 5.88 +- .21 | 6.81 +- .16 | 6.66 +- .31
| | | |
Breadth | | | |
of braincase | 8.54 +- .23 | 8.52 +- .12 | 9.84 +- .38 | 9.52 +- .20
| | | |
Alveolar | | | |
length, | | | |
upper molars | 2.98 +- .08 | 3.01 +- .08 | 3.20 +- .09 | 3.24 +- .10
--------------+------------+-------------+-------------+--------------

INDIVIDUAL VARIATION

Length of tail varied more than any other measurement used by me in taxonomic comparisons. Clark (1941:298), Hoffmeister (1951:16), and Van Gelder (1959:239) point out that external measurements generally are more variable than measurements of the cranium, probably because different techniques of measuring are employed by different collectors. As can be noted in table 3, females varied more than males.

In the 3520 specimens examined, an extra tooth was observed in only one (see Hooper, 1955:298). The left mandibular tooth-row of an adult male (USNM 71539) from Omentepec, Guerrero, is worn more than the right one. Irregularities in number of teeth and abnormalities in individual teeth seem to be rare in pygmy mice.

TABLE 3.--Individual Variation: Coefficients of Variation for
Dimensions of External and Cranial Parts in a Population of
B. Musculus and B. Taylori.

=====================+=========================+=========================
| Baiomys taylori | Baiomys musculus
+-------------------------+-------------------------
| Vic. (see page 595) | Vic. (see page 595)
Measurement | Altamira, Tamaulipas | El Salvador
+-----------+-------------+------------+------------
| 21 Males | 18 Females | 17 Males | 13 Females
| C. V. | C. V. | C. V. | C. V.
---------------------+-----------+-------------+------------+------------
| | | |
Total length | 3.0 | 4.7 | 4.9 | 3.7
Length of tail | 5.7 | 5.9 | 6.2 | 6.4
Length of body | 2.8 | 5.0 | 5.9 | 3.5
Length of hind foot | 3.7 | 3.4 | 3.0 | 4.1
Length of ear | 0.0 | 0.0 | 5.5 | 3.3
| | | |
Occipitonasal length | 2.2 | 2.7 | 1.8 | 2.3
Zygomatic breadth | 3.6 | 3.3 | 2.2 | 2.7
Interorbital breadth | 3.2 | 3.3 | 2.2 | 2.9
Incisive foramina | | | |
(length) | 3.8 | 4.6 | 2.5 | 3.2
Depth of cranium | 3.6 | 2.5 | 2.5 | 2.5
Alveolar length, | | | |
upper molars | 2.7 | 2.5 | 2.8 | 3.2
Postpalatal length | 3.1 | 4.7 | 2.1 | 2.9
Length of rostrum | 3.3 | 3.6 | 2.4 | 4.7
Breadth of braincase | 2.7 | 1.4 | 4.0 | 4.9
---------------------+-----------+-------------+------------+------------

The posterior margin of the bony palate varies from semicircular to nearly V-shaped. The suture between the nasals and frontals varies from V-shaped to truncate to W-shaped. The maxillary part of the zygoma varies from broad to slender in dorsoventral width in both species.

PELAGE AND MOLTS

There are three distinct pelages, juvenal, postjuvenal, and adult. The sequences of molt and change of pelage from the juvenal, to the postjuvenal, and from it to adult, are essentially as reported for _Peromyscus_ by Collins (1918:78-81; 1924:58-60) and Hoffmeister (1951:5). The juvenal pelage is uniformly dusky gray throughout except for the paler gray on the venter. In most juvenal mice, the yellow to ochraceous pigments of the subterminal bands are reduced or absent. Unlike _Peromyscus_, _Baiomys_ has bright brownish hairs on the head as the first evidence of the postjuvenal molt (see Figure 4, part a). Blair (1941:381) reports adult pelage in pygmy mice being evident first at an age of 46 days. Two of my juveniles born in captivity began the postjuvenal molt on the 38th and 40th days. The area of new hairs on the head spreads most rapidly posteriorly. New hair appears ventrally and laterally at the end of 46 days (see Figure 4, part b). Hair replacement proceeds more slowly after the "saddle back" stage (described in _Peromyscus_ by Collins, 1918:80) has been reached. That stage was reached in two pygmy mice at 52 days (see Figure 4, part c). Areas immediately posterior to the ears, in the scapular region, molt last. The postjuvenal pelage was seemingly complete in one captive pygmy mouse at the end of 60 days. Another captive failed to complete its growth of new pelage until two additional weeks had elapsed. Length of time required to molt in pygmy mice is about the same as that reported by Layne (1959:72) in _Reithrodontomys_.

If, after the postjuvenal molt, a distinct adult pelage is acquired it is difficult to separate it from the annual replacement of pelage in adults at the beginning of the rainy season. Adults of both species have been found in molt in all months of the year. To the north, in Texas, the pelage of winter-taken specimens is denser and slightly more reddish than that of specimens taken in spring and summer. In the two last mentioned seasons, the pelage is more uniformly gray. To the south, in Mexico, the pelage is heavy and long in most specimens taken in the rainy season. The percentage of specimens in molt immediately before the rainy season and immediately before the dry season is slightly higher than in specimens taken at other times of the year. The adult or seasonal molt (both loss of old pelage and growth of new) resembles that in _Peromyscus truei gilberti_, described by Hoffmeister (1951:6) as proceeding "posteriorly as a wave over the entire back." The new hair is slightly brighter than the old. Old adults are usually in ragged pelage regardless of season; possibly only one regular annual change of pelage occurs in most animals before they die. Only one case of melanism was observed among all the specimens of both species examined. It was a young male _B. t. taylori_, KU 35943, from 6 mi. SW San Geronimo, Coahuila, possessing black hairs throughout. Its hairs are longer and finer than those on specimens of comparable age and sex. No albino was found, although Stickel and Stickel (1949:145) record one--an adult male of _B. taylori_.

TAXONOMIC CHARACTERS AND RELATIONSHIPS

_External parts._--Length of body, foot, ear, and tail are useful when considered together in distinguishing species and subspecies. I found as Hooper (1952a:91) did that length of ear in combination with length of hind foot suffices to identify nearly all specimens to species, especially where the two species occur together.

_Pelage._--Color in adults is of especial value in subspecific determination; the manner in which it varies geographically is described on pages 609, 630.

_Skull._--Difference in occipitonasal length and zygomatic breadth, both having low coefficients of variation, are useful in separating species, especially where they are sympatric. Shape of presphenoid, nasals, interparietal, frontoparietal sutures, and length and degree of the openings of the incisive foramina are useful in delimiting subspecies. The rostrum of _B. taylori_, in front of the frontonasal suture, is deflected three to five degrees ventrally in 85 per cent of the adults examined, and in _B. musculus_ is less, or not at all, deflected.

_Teeth._--Alveolar length of the upper and lower molar tooth-rows aids in distinguishing fossil and Recent species, and to a lesser degree in delimiting subspecies. Occlusal pattern is useful in estimating the relationship of fossil and living species. Degree of development of the mesostyle, mesostylid, mesoloph, and mesolophid have been useful in determining relationship between fossil and living species as well as useful in separating the living species. Rinker (1954:119) and Hooper (1957:48) have shown the degree of variation in dental patterns in _Peromyscus_, _Sigmodon_, and _Oryzomys_, mice thought to be closely related to _Baiomys_. In pygmy mice, however, the dental patterns are relatively constant. The lophs and styles are subject to some geographic variation but, nevertheless, are useful in estimating relationships.

A. _Baiomys musculus brunneus_, adult, female, No. 30182 KU,
Potrero Viejo, 1700 feet, Veracruz.

B. _Baiomys taylori analogous_, adult, female, No. 36761 KU,
2 mi. N Ciudad Guzman, 5000 feet, Jalisco.]

_Hyoid apparatus._--Shape and, to a lesser extent, size of the hyoid apparatus differentiate nearly all specimens of _B. taylori_ from all those of _B. musculus_. The hyoid of _B. taylori_ differs from that of _B. musculus_ principally in the shape of the basihyal. It possesses an anteriorly pointed entoglossal process in _B. musculus_, and is not rounded to completely absent as in _B. taylori_ (see Figure 5). The shoulders of the basihyal protrude anteriorly in _B. musculus_, and are not flattened as in _B. taylori_. The total length was measured in a sample of 55 basihyals of _B. musculus_, and was compared to the total length of a sample of 80 basihyals of _B. taylori_. The means of the two samples differ significantly at the 95 per cent level; the mean plus two standard errors of _B. musculus_ and _B. taylori_, are, respectively, 2.43 +- .02; 2.18 +- .03. There is sufficient overlap of the samples (mean plus one standard deviation of _B. musculus_ and _B. taylori_, respectively: 2.43 +- .15; 2.18 +- .15) to make the total length of the basihyal of only secondary importance in distinguishing species, but shape and total length of the basihyal, when considered together, serve to identify all specimens to species. When length of the basihyal is plotted against occipitonasal length (see Figure 6), all specimens studied, regardless of age or geographical origin, were separated at the level of species. The hypohyals of _B. taylori_ seemingly remain distinct throughout life; those of _B. musculus_ completely fuse in some adults. The ceratohyals are highly variable in shape and of little taxonomic use.

The degree of geographic variation in shape of basihyal is not great. Specimens of _B. musculus pallidus_ from 1 km. NW Chapa, Guerrero, have a small indentation on the anteriormost part of the entoglossal process. The shoulder of the basihyal is directed less forward in specimens of _B. taylori taylori_ from 6 mi. N, 6 mi. W Altamira, Tamaulipas, than in other specimens of the species. The variations observed seemed not to be clinal.

According to White (1953:548) the hyoid, like the baculum (Burt, 1936:146), is little influenced by changes in external environment and may serve to clarify intergeneric relationships. Hyoids of both species of _Baiomys_ are smaller than hyoids of all subgenera of _Peromyscus_. In shape, the hyoids of _Baiomys_ resemble those of _Ochrotomys nuttalli_ (as explained on page 605, _Ochrotomys_ is here accorded generic, instead of subgeneric, rank). In size, the hyoid of both species of _Baiomys_ resembles that in _Reithrodontomys_. Sprague (1941:304) reports a resemblance in shape between the ceratohyals of _Baiomys_ and _Reithrodontomys_. The thyrohyals differ from those of _Reithrodontomys_, being less boot-shaped, and having a slight terminal expansion as in _Ochrotomys_ (see Sprague, _loc. cit._). In shape, the large basihyal of _Onychomys_ resembles the smaller one of _B. musculus_. The basihyal of _Oryzomys_ lacks the entoglossal process present in _Baiomys_. On the basis of shape of hyoid, _Baiomys_ seems to be most closely related to _Ochrotomys_.

A. _B. musculus brunneus_, adult, No. 24336 KU, 3 kms.
W Boca del Rio, 10 feet, Veracruz.

B. _B. taylori taylori_, adult, No. 35937 KU, 6 mi.
SW San Geronimo, Coahuila.]

_Baculum._--Of _Baiomys_, 166 bacula were processed, using the method of White (1951:125), and studied. They provide characters of taxonomic worth at the level of species and aid in evaluating generic relationships.

The baculum of _B. taylori_ differs from that of _B. musculus_ in: shaft narrow; wings anterior to base projecting dorsolaterally instead of anteriorly; anterior part knob-shaped having indentation at tip, instead of anterior part spatulate-shaped (in some) to knob-shaped (see Figure 7), without indentation; significantly shorter (see Table 4).

TABLE 4.--Length of Bacula

==============+===========+=========+==========+===========+==========
| Number of | Average | 3 x | 1 |
Species | specimens | length | standard | standard | Range
| | | error | deviation |
--------------+-----------+---------+----------+-----------+----------
_B. taylori_ | 108 | 2.535 | .078 | .274 | 2.00-3.12
| | | | |
_B. musculus_ | 58 | 3.324 | .090 | .233 | 2.80-3.88
--------------+-----------+---------+----------+-----------+----------

In each of the two species, individual and geographic variation in the baculum is slight; its length varies insignificantly according to age. Excluding juveniles contained in Table 4, but including young and subadults, only three bacula of _B. taylori_ were longer than 3 mm., and only one baculum of _B. musculus_ (a young) was shorter than 3 mm. The total length of the baculum, considered together with its shape, serves to identify to species all specimens examined by me.

The bacula of both species of _Baiomys_ were compared with bacula of _Akodon_, _Scotinomys_, _Holochilus_, _Oryzomys_, _Zygodontomys_, _Reithrodontomys_, _Thaptomys_, and _Calomys_ and illustrations of bacula by Blair (1942:197, 200) of _Peromyscus_ (subgenera _Peromyscus_, _Haplomylomys_, _Podomys_), _Ochrotomys_, and material at the University of Kansas Museum of Natural History of _Megadontomys_. Shape of baculum most resembled that of _Ochrotomys_ and _Calomys_. The bacula of _Baiomys_, as pointed out by Blair (_op cit._:203), differ as much from those of the genus _Peromyscus_ as do the bacula of _Reithrodontomys_ and _Onychomys_. In size of baculum, _Baiomys_ resembles _Ochrotomys_. Blair (_op. cit._:202) pointed out that the length of the baculum of _B. taylori subater_ was contained in the length of the animal's body 20.3 times, and 24.2 times in the length of that of _Ochrotomys nuttalli_. The length of the baculum of _B. musculus_ (average of 58 specimens without regard to subspecies) is contained in the length of the body (of specimens from which the bacula were removed) 22.7 times, a figure approaching that in _Ochrotomys_. When bacula of both species of _Baiomys_ were compared to those of _O. nuttalli_, bacula of _B. musculus_ were found to most closely resemble those of _O. nuttalli_. The baculum of a single specimen of _Calomys_ (_C. laucha_) was contained in the length of the body 15.5 times. In general shape, as well as in possession of an anterior knob and the position of the expanded posterior wings, the baculum of _C. laucha_ resembles the baculum of _Ochrotomys_ and _Baiomys musculus_.

Blair (_op. cit._:201) considers generic _versus_ subgeneric rank for _Ochrotomys_, and on the basis of studies of the phallus Hooper (1958:23) stated that "it is clear that _nuttalli_ should be removed from _Peromyscus_ and should be listed as _Ochrotomys nuttalli_ (Harlan)." I agree with Hooper (_loc. cit._) and point out that on the basis of the baculum, there is less of a hiatus between _Baiomys_ on the one hand, and _Ochrotomys_ and _Calomys_ on the other hand, than there is between any one of those three genera and _Peromyscus_.

White (1953:631) reported that the baculum of chipmunks might indicate relationships more clearly than do skulls and skins. He thought that skulls might more quickly than bacula reflect the habitus of the animal. The resemblance in cranial morphology between _Peromyscus_ and _Baiomys_ is judged to be the result of such a convergence of habitus and the baculum in _Baiomys_ is thought to reflect relationships more accurately than does the skull.

_Auditory ossicles._--Examination of a number of auditory ossicles of _Baiomys_ reveals constant interspecific differences in the malleus and incus. There is only slight individual variation, slight variation with age, and no secondary sexual variation. In _Baiomys taylori_ the orbicular apophysis of the malleus (see Figure 8, A) is rounded to nearly ovoid; the anterior process is pointed, and the neck is short, being slightly recurved. The body of the incus is round and the short process is elongate. The sides of the long limb of the incus are nearly parallel. The lenticular process is relatively large. The posterior and anterior crus of the stapes are bowed, and the muscular process is either absent or much reduced.

In _Baiomys musculus_, the orbicular apophysis of the malleus (see Figure 8, B) is round to oblong, and less ovoid than in _B. taylori_; the anterior process is less acutely pointed than in _B. taylori_, and the neck is long, less recurved than in _B. taylori_. The body of the incus, though tending to be round, is more flattened, and the short process is knob-shaped, not elongated. The sides of the long limb of the incus are not parallel. The lenticular process is, relative to the size of the incus, small. The posterior and anterior crus of the stapes are more nearly straight than in _taylori_. A prominent muscular process occurs on the posterior crus.

The auditory ossicles of representative species of all the subgenera of _Peromyscus_ were studied as were the ossicles of _Onychomys_, _Ochrotomys_, _Oryzomys_, _Akodon_, _Thaptomys_, _Zygodontomys_, _Calomys_, _Reithrodontomys_, and _Holochilus_.

A. _B. taylori analogous_, adult, female, No. 28104 KU, 4 kms.
ENE Tlalmanalco, 2290 meters, Estado de Mexico.

B. _B. musculus pallidus_, adult, male, No. 28346 KU, Cahuilotal,
Sacacoyuca, 960 meters, Guerrero.]