_Comparisons._--For comparisons with _B. t. canutus_, see account of that subspecies. From _B. t. analogous_, _B. t. allex_ differs in: external and cranial dimensions less; dorsal coloration paler; tail and ears paler and less hairy; dorsum and belly paler; dorsal and ventral parts of forefeet and hind feet paler; median parts of incisive foramina less constricted on either side of midline and wider open laterally; interparietal larger in relation to skull; interorbital breadth greater relative to occipitonasal length. _B. t. allex_ differs from _B. t. paulus_ as follows: dorsum gray with yellowish-brown wash rather than fawn to buff; tail unicolored in most series, less hairy; hind feet flesh-colored to light sooty, rather than whitish; rostrum slightly longer relative to occipitonasal length; incisive foramina differ from those of _paulus_ in much the same way as from _analogous_. _Remarks._--Osgood (1909:255-256) dismissed as taxonomically unimportant the differences in color of pelage and size of cranium that he observed between the specimens from Colima (City), Colima, representative of _allex_ and those representing _paulus_ and chose to synonomize _allex_ with _paulus_. The differences that Osgood (_loc. cit._) deemed "... scarcely worthy of recognition ...," are, in fact, not only worthy of recognition, but also important in an understanding of the evolution of _Baiomys taylori_ (see speciation p. 659). Recently, I (1958b:17-18) studied ten specimens from Colima (City), Colima, and chose to regard _Peromyscus [= Baiomys] allex_ as a subspecies. I suggested (_loc. cit._) that the geographic range of _B. t. allex_ might encompass the southern part of Nayarit, and western Jalisco. Subsequent study of specimens from these areas reveals that the populations there are referable to _allex_. Most of the specimens obtained from these areas, however, merit special comment. In color of pelage, those populations from south of the Rio Grande de Santiago and northwest of Guadalajara (4 mi. SE Ahuacatlan; 1 mi. E Ixtlan; Etzatlan) show evidence of intergradation with _paulus_ to the east and south (Magdalena, Tequila, and Tala, Jalisco), and with populations more closely adjacent to the south bank of that river. Intergradation is indeed complex in this area. Specimens from some localities seem to be intergrades between _allex_ and _paulus_; from other localities, some specimens are referable to _allex_, and the others to _paulus_; from still other localities, all specimens are referable to _allex_.