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_, _ZyG.o.dontomys_, _Reithrodontomys_, _Thaptomys_, and _Calomys_ and ill.u.s.trations 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 k.n.o.b 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 s.e.xual 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 k.n.o.b-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_, _ZyG.o.dontomys_, _Calomys_, _Reithrodontomys_, and _Holochilus_.

[Ill.u.s.tration: FIG. 8. Lateral views of auditory ossicles. 20.

A. _B. taylori a.n.a.logous_, adult, female, No. 28104 KU, 4 kms.

ENE Tlalma.n.a.lco, 2290 meters, Estado de Mexico.

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

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 k.n.o.blike 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 _ZyG.o.dontomys_, size and shape of the ossicles differ greatly from those of _Baiomys_.

In the genus _Peromyscus_, only _Peromyscus florida.n.u.s_ (subgenus _Podomys_) possesses a k.n.o.blike 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 s.p.a.ce 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.

[Ill.u.s.tration: FIG. 9. Geographic distribution of the genus _Baiomys_. Black area shows where the two species occur together.

Black dot (Acultzingo, Veracruz) shows locality where _Baiomys taylori_ occurs within the range of _B. musculus_, but _B. musculus_ is not known to occur at that locality.]

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, Michoacan, Mexico, Morelos, Puebla, and central Veracruz, southeastward to western Nicaragua, but unknown from southern Veracruz, Tabasco, and the Yucatan Peninsula (see Figure 10); occurs princ.i.p.ally 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 p.r.o.nounced; basihyal having anterior pointed entoglossal process, shoulders of basihyal protruding anteriorly (characteristic of all age categories); baculum having broader shaft, spatulate to k.n.o.b-shaped tip, wings at base projecting anteriorly; baculum more than 3 millimeters long; short process of incus k.n.o.b-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 alt.i.tudes average larger than those from low elevations, except where the two species are sympatric. There the Southern Pygmy Mouse is uniformly larger, regardless of alt.i.tude.

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.

[Ill.u.s.tration: FIG. 10. Distribution of _Baiomys musculus_. Known localities of occurrence are represented by circles and black dots; the former denote localities that are peripheral (marginal) for the subspecies concerned.

1. _B. m. brunneus_ 2. _B. m. grisescens_ 3. _B. m. handleyi_ 4. _B. m. infernatis_ 5. _B. m. musculus_ 6. _B. m. nigrescens_ 7. _B. m. pallidus_ 8. _B. m. pullus_]

_Natural History_

_Habitat and numbers._--In Veracruz, Dalquest obtained the southern pygmy mouse in stands of tall gra.s.s (_Spartina?_) in sandy loam soil bordering, and in, dense vegetation; Davis (1944:394) found the species living in dense stands of gra.s.ses 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 spa.r.s.e gra.s.s and rocky hillside bounding a stream and in litter below shrubs on the floor of a nut-palm forest; in Michoacan, these mice were taken in cane gra.s.s, 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 spa.r.s.e gra.s.ses 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 Izucar de Matemores, Puebla, along a stream in heavy gra.s.s 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 Izucar 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 Izucar 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.