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_. [Illustration: FIG. 8. Lateral views of auditory ossicles. x 20. 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.]