| Number of specimens | Average length | 3 × standard | 1 standard | Range | |
| 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.