Sonoran and Chihuahuan deserts. _B. taylori_ seems to be morphologically more specialized for life in an arid grassland than was _B. sawrockensis_. The geographic range of ancestral pygmy mice possibly extended farther south in late Pliocene time than the range of _B. musculus_ does now. Anyhow, _B. sawrockensis_ of the early late Pliocene dwelt in a more mesic type of habitat than _B. musculus_ does, and such habitat may have existed from the Pacific lowlands of Central America to the Caribbean lowlands of northern South America (see Duellman, 1958:136, and Dunn, 1940:156) during late Pliocene times. An ancestral stock of hesperomine mice, not greatly different from _Baiomys_, may have emigrated from the North American continent into South America across the continuous land connection, which Simpson (1950:395) suggests was formed in the Chapadmalalan age (= Blancan age of North American terminology). The length of time of interchange of genes between northern and southern populations of mice across the Central American land connection probably was brief. Duellman (_op. cit._:129) pointed out that once the Panamanian portal was closed, the warm counter equatorial current, El Nino, combined with the uplifting of the Andes, began to produce heavy rain forests in Central America and northern South America in late Pliocene or early Pleistocene times. These forests presumably isolated the stock in North America from that in South America where the latter probably evolved rapidly into kinds that differed from one another and from _Baiomys_ in shape of body, type of pelage, and shape of skull. Internal structures such as hyoid apparatus, auditory ossicles, and baculum remained almost unchanged, as for example in _Calomys_ now living in South America. The present resemblance in internal morphological features between it and _Baiomys_, I judge, reflects taxonomic relationships more accurately than do shape and conformation of body and skull that seem to respond more rapidly to external environmental changes. The cranial characters distinguishing _Baiomys musculus_ from _Calomys laucha_ are as follows: posterior lacerate foramina between second, rather than first, upper molars; parapterygoid fossa shallower; mesopterygoid fossa as wide or wider, instead of narrower, than parapterygoid processes; burr for attachment of superficial masseter muscle hypertrophied instead of well-developed. In other cranial characters studied, the two genera closely resemble each other. Such similarities of crania between _Calomys_ and _Baiomys_ may reflect convergence, but the total of internal and external morphological characters shared, I think reflects true relationships.