ontogeny in both _Pliosaccomys_ and the Geomyini become eventually united, with wear, into a single column. Based upon the foregoing evidence, it would seem likely that the Geomyini evolved from an early Pliocene (perhaps late Miocene) Dikkomyini ancestor that had evolved the specializations of M1 and M2 that characterize its relative, _Pliosaccomys_, but had not also evolved the specializations of m1 and m2 that distinguish _Pliosaccomys_. Therefore, the ancestor of the Geomyini differed from the _Pliosaccomys_-Thomomyini lineage in its retention, unmodified, of the primitive patterns in m1 and m2 that characterized the earliest known Geomyines (_Dikkomys_). The same patterns are preserved in m1 and m2 of its modern descendents, the living Geomyini. In the _Pliosaccomys_-Thomomyini lineage the pattern of m1 and m2 are entirely different, as described above. The earliest record of the Geomyini is the extinct genus _Pliogeomys_ (see Fig. 6) in the latest Hemphillian (middle Pliocene) and earliest Blancan (late Pliocene). _Pliogeomys_ is more primitive than any modern genus of the Geomyini, seems to have been a late survivor of the primitive stock, but was itself probably a collateral lineage and not on the direct line of descent. The cheek teeth in _Pliogeomys_ are rooted and less hypsodont than in the late Pliocene examples of the modern genera, and the anterior enamel plate of the lower molars shows no indication of reduction, as would be expected if _Pliogeomys_ were in the direct line of evolution. Separation of _Pliogeomys_ from the main stem of the Geomyini probably occurred after several specializations had already been achieved by the Geomyini. Two inheritances might have been grooving on the upper incisors and some reduction in amount of enamel on the sides of the cheek teeth. The dentine tracts on the sides of the cheek teeth of _Pliogeomys_ are narrow (see Fig. 7A) and barely separate the enamel blades and there is no discernible reduction in the anterior enamel blades on its lower molars. Those blades evidently were lost in the main lineage before the Pleistocene radiation of the living genera took place. _Pliogeomys_ is in an intermediate stage in evolution, and was not so advanced as was the main lineage at the time _Pliogeomys_ died out. Its structure does provide clues as to phyletic development that took place in the main lineage.