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.