_Teeth._--Alveolar length of the upper and lower molar tooth-rows aids in distinguishing fossil and Recent species, and to a lesser degree in delimiting subspecies. Occlusal pattern is useful in estimating the relationship of fossil and living species. Degree of development of the mesostyle, mesostylid, mesoloph, and mesolophid have been useful in determining relationship between fossil and living species as well as useful in separating the living species. Rinker (1954:119) and Hooper (1957:48) have shown the degree of variation in dental patterns in _Peromyscus_, _Sigmodon_, and _Oryzomys_, mice thought to be closely related to _Baiomys_. In pygmy mice, however, the dental patterns are relatively constant. The lophs and styles are subject to some geographic variation but, nevertheless, are useful in estimating relationships. [Illustration: FIG. 5. Ventral view of hyoid bones. x 18. A. _Baiomys musculus brunneus_, adult, female, No. 30182 KU, Potrero Viejo, 1700 feet, Veracruz. B. _Baiomys taylori analogous_, adult, female, No. 36761 KU, 2 mi. N Ciudad Guzman, 5000 feet, Jalisco.] _Hyoid apparatus._--Shape and, to a lesser extent, size of the hyoid apparatus differentiate nearly all specimens of _B. taylori_ from all those of _B. musculus_. The hyoid of _B. taylori_ differs from that of _B. musculus_ principally in the shape of the basihyal. It possesses an anteriorly pointed entoglossal process in _B. musculus_, and is not rounded to completely absent as in _B. taylori_ (see Figure 5). The shoulders of the basihyal protrude anteriorly in _B. musculus_, and are not flattened as in _B. taylori_. The total length was measured in a sample of 55 basihyals of _B. musculus_, and was compared to the total length of a sample of 80 basihyals of _B. taylori_. The means of the two samples differ significantly at the 95 per cent level; the mean plus two standard errors of _B. musculus_ and _B. taylori_, are, respectively, 2.43 +- .02; 2.18 +- .03. There is sufficient overlap of the samples (mean plus one standard deviation of _B. musculus_ and _B. taylori_, respectively: 2.43 +- .15; 2.18 +- .15) to make the total length of the basihyal of only secondary importance in distinguishing species, but shape and total length of the basihyal, when considered together, serve to identify all specimens to species. When length of the basihyal is plotted against occipitonasal length (see Figure 6), all specimens studied, regardless of age or geographical origin, were separated at the level of species. The hypohyals of _B. taylori_ seemingly remain distinct throughout life; those of _B. musculus_ completely fuse in some adults. The ceratohyals are highly variable in shape and of little taxonomic use.