New dental specimens described here from the Barstovian CC South and Flint Creek localities of western Montana are determined to be of two geomyid rodent species, Lignimus transversus and Mojavemys sp. These species were known previously from sparse material and the new samples aid the clarification of their biogeography and evolution. The presence of Lignimus transversus at both localities extends the range of that species to the north. In Mojavemys, a cline of increasing tooth size from south to north is analogous to clines in extant geomyids that correlate with nutritional or climatic gradients. Comparisons of various dental morphologic features of the new material and of the previously described populations of both genera reveal that the examined traits as well as the populations themselves were not genetically linked, and suggest mosaic evolution operated in these species. The differences in dental morphology between the various depositional basins are consistent with the idea of a tectonic influence on mammalian evolution.