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.
Fig. 7. Dorsal view of bacula. × 16.
A. B. musculus brunneus, adult, No. 24336 KU, 3 kms. W Boca del Río, 10 feet, Veracruz.
B. B. taylori taylori, adult, No. 35937 KU, 6 mi. SW San Gerónimo, 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]).