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

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 México, 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.

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 reëntrant 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 proödont, more retrodont.

From _B. kolbi_, _B. sawrockensis_ differs in: crowns of molars narrower; incisors less proödont; 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 proödont; 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, proödont; 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 proödont; 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 proödont; 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 proödont; 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, proödont; 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 proödont.

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 reëntrant valley broad and extending farther across crown of tooth; incisor smaller, and more proödont; cingular ridges of m1 and m2 less well-developed.

From _B. taylori_, _B. kolbi_ differs in: molars larger, more depressed; incisor shorter, more proödont; m3 smaller relative to m1 and m2; external reëntrant 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, proödont; 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 proödont.

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

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

From _B. taylori_, _B. brachygnathus_ differs in: incisor more slender, shorter, more proödont; 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 proödont 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 reëntrant 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 proödont; 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 proödont 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 proödont 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 México, 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 Gerónimo, 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.

_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_.

_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 × | 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_.

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

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

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

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

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

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

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

Genus =Baiomys= True

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

_Diagnosis._--Size small (total length in adults, 93-135); tail shorter than head and body; hind foot in adults 12-17; ears small (8-12) and rounded; upper parts blackish sepia to ochraceous-buff; underparts slaty gray to white or pale buffy; eyes small; hind feet having six plantar pads, soles nearly naked except for some hairs on anterior parts of soles and anteriorly to base of toes and between toes; occipitonasal length of skull in adults, 17.0-21.5; zygomatic breadth, 9.0-11.5; coronoid process of mandible well developed, strongly recurved; ascending ramus of mandible short and erect; anterior palatine foramina (incisive foramina) long, usually terminating posterior to plane of the front of first molars; posterior palatine foramina nearly opposite middle of M2; interorbital space wide relative to widest part of frontals; nasals projecting only slightly over incisors; condyle terminal; upper incisors relatively heavy; primary first fold of M3 obliterated at an early stage of wear; major cusps of upper and lower anteriormost two molars alternating, more so in m1-m2 than in M1-M2, dental formula I/i, 1/1; C/c, 0/0; P/p, M/m, 3/3 = 16.

For distribution of the genus, see Figure 9.

SYSTEMATIC ACCOUNT OF SPECIES AND SUBSPECIES

=Baiomys musculus=

Southern Pygmy Mouse

(Synonymy under subspecies)

_Type._--_Sitomys musculus_ Merriam, Proc. Biol. Soc. Washington, 7:170, September 29, 1892.

_Range._--Southern Nayarit, Michoacán, México, Morelos, Puebla, and central Veracruz, southeastward to western Nicaragua, but unknown from southern Veracruz, Tabasco, and the Yucatán Peninsula (see Figure 10); occurs principally in the arid upper and lower divisions of the Tropical Life-zone.

_Characters for ready recognition._--Unless otherwise noted, characters are usable only for the two age-categories of adult and old adult. Differs from _B. taylori_ in: hind foot 16 millimeters or more; occipitonasal length, 19 millimeters or more; zygomatic breadth, 10 millimeters or more; rostrum not deflected ventrally at frontoparietal suture but, instead, curving gradually toward anteriormost point of nasals; cingular ridges and secondary cusps on teeth more pronounced; basihyal having anterior pointed entoglossal process, shoulders of basihyal protruding anteriorly (characteristic of all age categories); baculum having broader shaft, spatulate to knob-shaped tip, wings at base projecting anteriorly; baculum more than 3 millimeters long; short process of incus knob-shaped rather than attenuate; muscular process of posterior crus of stapes prominent.

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

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

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

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

_Natural History_

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

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

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

_Enemies and food._--Owl pellets (thought to be those of a barn owl, _Tyto alba_) from within the geographic range of _B. musculus_, from 6