The subfamily Geomyinae is made up of three groups, recognized taxonomically for the first time in this account as tribes--Dikkomyini, Thomomyini, and Geomyini (for full discussion of classification, see previous account). The phylogeny proposed by me is illustrated in Figure 3. The tribe Dikkomyini is characterized by generalized and primitive features that together form the basic structural foundation of the subfamily. Evolution within the Dikkomyini resulted in the acquisition and perfection of fossorial adaptations. The Thomomyini and Geomyini are considerably more specialized than the ancestral Dikkomyini from which they evolved. The Geomyini are clearly more specialized than the Thomomyini, suggesting closer affinity between the Thomomyini and the Dikkomyini than between the Geomyini and the Dikkomyini. The specializations in the dentition and the associated changes in the skull of the Thomomyini and Geomyini permit more efficient mastication of fibrous vegetation. Along with these specializations, fossorial adaptations inherited from the Dikkomyini are retained without noteworthy modification. _Dikkomys_, the earliest known genus of the tribe Dikkomyini, can be taken as a starting point of evolution for the subfamily Geomyinae. The Pliocene genus _Pliosaccomys_ is the only other known geomyine having primitive features closely resembling those of _Dikkomys_. The relatively close but previously unrecognized relationship between _Dikkomys_ and _Pliosaccomys_ can be understood when patterns of wear on the occlusal surfaces of the cheek teeth are taken into account. It appears that _Pliosaccomys_ descended from _Dikkomys_-like stock, if not _Dikkomys_ itself. Although _Dikkomys_ is towards the beginning of this phyletic sequence and _Pliosaccomys_ towards the end of the sequence, the primitive features shared by the two provide a generalized morphotype for the subfamily Geomyinae. In the molariform dentition, an almost complete series of stages of wear in _Pliosaccomys_ has been preserved, and those of _Dikkomys_ can be reconstructed with reasonable accuracy from those that are known (see Fig. 4): (1) In the initial stage of wear in _Dikkomys_ the anterior and posterior columns are separated by an intervening valley (Fig. 4A), and the occlusal surface of each column bears a loph of dentine surrounded by a ring of enamel: protoloph on the anterior column and metaloph on the posterior column of the upper teeth (protolophid and hypolophid in corresponding positions in the lower teeth). Actually this stage is not preserved in the known material of _Dikkomys_, but does occur in both geomyines and entoptychines in all stages of evolution, and it must have also occurred in _Dikkomys_ in order for the next two stages, which are preserved, to have developed.