The principal trend of evolution in these semi-fossorial rodents was toward more complete fossorial adaptation, and the pronounced fossorial features characteristic of the modern pocket gophers were perfected. This trend continued in response to the intense selection pressures in this arid environment. The principal structural characters effected were in the postcranial anatomy, especially in the skeletal and muscular systems. Consequently, it is not surprising that in skull and dentition, _Pliosaccomys_ differs but little from _Dikkomys_. Therefore, most of the basic structural specializations so far developed for subterranean existence probably had evolved by the time geomyines moved back north in the early Pliocene. Both modern lineages, the tribes Thomomyini and Geomyini, have essentially the same fossorial features, and it seems unlikely that these features were acquired independently in the relatively short period of time available to them after their divergence; probably they were inherited from a common ancestor. These probabilities indicate that the evolution of the fossorial specialization was in the later phyletic development of the tribe Dikkomyini. Plio-Pleistocene radiation of Geomyini Unlike the lineage of the Thomomyini that remained essentially rectilinear through out its history, the Geomyini in the late Pliocene and the early Pleistocene underwent adaptive radiation in a degree comparable to the earlier radiation of the Entoptychinae, and all of the later history of the tribe is dominated by the radiation--the resulting structural diversity. At least four lineages were produced by the Plio-Pleistocene radiation (see Fig. 6); each originated at essentially the same time (late Pliocene) presumably from the same ancestral stock. Each of these lineages within the Geomyini has given rise to one of the four modern genera: _Zygogeomys_, _Geomys_, _Orthogeomys_, and _Pappogeomys_. [Illustration: FIG. 6. Plio-Pleistocene radiation of the Tribe Geomyini.] _Morphotype_ The immediate, unknown, ancestor probably lived on the Central Plateau of México. After the radiation began the ancestors of _Geomys_ and _Zygogeomys_ extended their ranges northward.