---------------------+---------------------+------------------- | Geomyidae | Geomyidae | _incertae sedis_ | _incertae sedis_ | | | | *_Gidleumys_ | *_Diplolophus_ | *_Diplolophus_ | *_Griphomys_ | *_Griphomys_ ---------------------+---------------------+------------------- Family | Family | Family Heteromyidae | Heteromyidae | Heteromyidae ---------------------+---------------------+------------------- * Denotes extinct genera. ** Winge included in his family Saccomyidae the "group" Gymnoptychine and the contained genus _Gymnoptychus_ Cope, 1873, which genus currently is placed in the family Eomyidae. The type of _Gymnoptychus_ Cope, 1873, is synonymous with _Ischyromys_ Leidy, 1856, and the valid name for the genus is _Adjidaumo_ Hay, 1899. _Pliosaccomys_, on the other hand, represents the terminal stages of a long trend that began with the _Dikkomys_-like Geomyinae of the early Miocene. In this lineage, the rate of evolution in the dentition and the skull was slow; therefore, the differences between early Miocene (_Dikkomys_) and middle Pliocene (_Pliosaccomys_) are not great and the two are united into the tribe Dikkomyini. The Dikkomyini is the ancestral geomyinen trunk from which the modern groups have diverged. The Pliocene ancestor of _Thomomys_ is unknown but probably resembled _Pliosaccomys_, with which it may have been a contemporary. _Thomomys_ is the least specialized of the modern Geomyinae, and, consequently, shows the most resemblance to the ancestral tribe. The specializations of _Thomomys_, however, clearly preclude its reference to the tribe Dikkomyini; therefore, it is set apart in the monotypic tribe Thomomyini. That tribe has not undergone an adaptive radiation comparable to that of the tribe Geomyini or that of the Entoptychinae in the early Miocene. Here, for the first time, _Thomomys_ is set apart in classification from the other living pocket gophers.