In both species of _Baiomys_, the most distinct subspecies, _B. t. allex_ and _B. m. musculus_, occur in the area where the two species are sympatric. Seven subspecies, or 44 per cent, occur either in or adjacent to the transverse volcanic zone. This area is the major area of active differentiation. Incipient subspecies are also evident in these areas. A secondary area of differentiation is indicated within the range of _B. musculus_ in Guatemala, El Salvador and Honduras. Three subspecies occur in this area (_grisescens, handleyi_ and _nigrescens_) and incipient subspeciation is in evidence there. ZOOGEOGRAPHIC POSITION Hooper (1949:25) regards _Baiomys_ as a member of the rodent fauna of the arid, western Sonoran region, whereas Hershkovitz (1958:609) suggests that _Baiomys_ is a nearctic-neotropical varicant (a kind that occurs in contiguous zoogeographic regions without our knowing in which region the taxon originated). The findings from my study do not contradict either of the above suggestions. Because of the close resemblance of _Baiomys_ to certain hesperomine mice of South America, it is postulated that _Baiomys_, in more primitive form than now, occurred farther south in past times than it does now. Fossils show that primitive stocks of the genus in late Pliocene or early Pleistocene times occurred also north of the present range of the genus. The belt in west-central Mexico between nearctic and neotropical regions is the current center of distribution of the genus and probably has been for a considerable time. [Illustration: FIG. 12. Averages of the occipitonasal lengths of skulls of adults at 19 localities of occurrence (solid symbols) of _Baiomys taylori_, and at 17 localities of occurrence (open symbols) of _Baiomys musculus_. Note that the occipitonasal length decreases from north to south in each of the two species, and that in the region where the two species occur together, west-central Mexico, _B. taylori_ is smallest and _B. musculus_ is largest. Average, extremes, number of specimens averaged (in italic type), and name of locality, from north to south for each species, are as follows: _Baiomys taylori_