H-pattern is formed in the initial stages of wear; therefore, in the early Pliocene ancestor, presently unknown in the fossil record, the H-pattern probably was present. Even so, the ancestor of the Geomyini and that of _Pliosaccomys_ probably were closely allied otherwise, and both probably had attained the highly specialized fossorial adaptations characterizing all modern pocket gophers, before the divergence of _Pliosaccomys_ and the Geomyini took place. The evidence points to a major divergence of the geomyines that lived in the latest Miocene or the early Pliocene (probably the latter) and that gave rise to the two modern lineages, Thomomyini and Geomyini (see Fig. 3). One, the most primitive of the two, gave rise to the Thomomyini lineage that eventually evolved into _Thomomys_. _Pliosaccomys_ is closely allied to the ancestry of this lineage, although it is probably not the actual ancestor, as mentioned previously. Aside from the aforementioned specializations of the first and second molars, the features of the Thomomyini are less advanced than in the other specialized lineage (tribe Geomyini). Primitive traits retained in the tribe Thomomyini (and also characteristic of the ancestral tribe Dikkomyini) are: (1) Small size, in general no larger than the ancestral morphotype; (2) lack of grooving on the upper incisor (although a slight rudimentary groove is developed rarely in some living species); (3) retention of anterior and posterior enamel plates in lower and upper cheek teeth; (4) premolars having widely open re-entrant folds; (5) smooth and generalized skull lacking marked angularity, regosity or cresting (neither the sagittal nor the lambdoidal crest are ordinarily well developed except in _Thomomys bulbivorus_); (6) forefoot small, less modified for digging than in the Geomyini. [Illustration: FIG. 4. Drawings of the molariform dentitions of _Dikkomys_ and _Pliosaccomys_ (Tribe Dikkomyini) depicting the patterns of wear on the occlusal surfaces. Ontogenetically, the stages of wear are arranged from left to right in each row. Stages not represented by actual specimens have been carefully reconstructed from information provided by known stages in the sequence of wear and the dentitions of other geomyines. × 5. A-D. _Dikkomys woodi_, right lower tooth-row, including p4-m3. Patterns based on No. P26284 (FMNH) from Upper Rosebud (Middle Miocene), Shannon Co., South Dakota (B above). E-H. _Pliosaccomys dubius_, left upper tooth-row, including P4-M2 (M3 unknown). Patterns based on Nos. 1798 and 1799 (LAM) from Smiths Valley (Middle Pliocene), Lyon Co., Nevada. I-L. _Pliosaccomys dubius_, right lower tooth-row, including p4-m3. Patterns based on Nos. 1796 (holotype), 1804, and 1806 (LAM) from Smiths Valley (Middle Pliocene), Lyon Co., Nevada. ]