_B. minimus_ seemingly is more closely related to _B. sawrockensis_ and _B. musculus_ than to the other described species. The development of the cingular ridges leads one to suspect that _B. minimus_ was the ancestor of _B. musculus_. _B. minimus_ may have been derived from a _sawrockensis_-like stock and probably gave rise to _B. musculus_. Hershkovitz (1955:643-644) suggests that "... primitive brachydont, buno-mesolophodont cricetines have survived ... in forested parts of the range," whereas "... the progressive branch of cricetines with mesoloph absent or vestigal, has become increasingly specialized for life in open country and a diet of grasses." Species of the genus _Baiomys_ can be divided into two morphological groups. One group, composed of _B. sawrockensis_, _B. minimus_, and _B. musculus_, includes those species, the teeth of which were relatively brachydont and had prominently developed cingular ridges (ectolophids or mesolophids) or, at least, showed some development of these ridges. _B. sawrockensis_ probably lived in semi-wooded to shrubby habitats. According to Hibbard (1953:409), "The Saw Rock Canyon fauna lived in that area at a time when conditions were comparable to the conditions at the time the Rexroad fauna lived." The conditions in which the Rexroad fauna lived are discussed by Hibbard (1941:95). Presumably, there were at least some well-wooded situations, and the climate was warm. _B. sawrockensis_ probably inhabited denser vegetation than did _B. minimus_ or than does _B. musculus_. The teeth of the second group (_B. kolbi_, _B. rexroadi_, _B. brachygnathus_, and _B. taylori_) lack cingular ridges or have them much reduced and have more hypsodont molars. The three fossil species probably inhabited relatively open grassland. This assumption is based largely on the known habitat of _B. taylori_ (see page 632). The suggested grouping, based on supposed similarities in niches inhabited by the extinct species, does not necessarily indicate degree of relationship. _B. taylori_ probably was not derived from an ancestor like _B. rexroadi_ or _B. kolbi_, although, in certain characters, the three species resemble one another. _B. kolbi_ and _B. rexroadi_ were already specialized in Blancan times, probably for living on grassland. _B. taylori_ shows only a slight advance in specialization of molar structures compared to either of the aforementioned species but is slightly smaller and does have longer and more recurved incisors. If only morphological criteria of lower jaws were considered, without recourse to other data derived from the study of many samples of populations of the living species, time alone might account for the differences among _B. taylori_, _B. rexroadi_, and _B. kolbi_. The available evidence (see page 658) suggests, however, that _B. taylori_ was derived from the _B. sawrockensis_-_B. minimus_-_B. musculus_ line.