[Illustration: FIG. 6. Relationship of length of basihyal to occipitonasal length of skull. Black symbols, all below the curved line, represent measurements of _B. taylori_; open symbols, all above the curved line, represent measurements of _B. musculus_.] 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_. [Illustration: FIG. 7. Dorsal view of bacula. x 16. 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