Part 7

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.]

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.