A and E. _Geomys bursarius majusculus_, No. 2948 (KU), Douglas Co., Kansas. Right upper (A) including DP4-M3; lower left (E) including dp4-m3. B and F. _Pappogeomys bulleri burti_, No. 100444 (KU), 10 mi. NNW Barra de Navidad, Jalisco. Right upper (B) including P4-M3; right lower (F) including p4-m3 (both P4 and p4 with unworn enamel caps). C and G. _Pappogeomys bulleri albinasus_, No. 31044 (KU), 10 mi. S and 8 mi. W Guadalajara, Jalisco. Right upper (C) including P4-M3; right lower (G) including p4-m3. D and H. _Pappogeomys bulleri albinasus_, No. 31002 (KU), W side La Venta, 13 mi. W and 4 mi. N Guadalajara, Jalisco. Right upper (D) including P4-M3; right lower (H) including p4-m3. I and J. _Thomomys talpoides bridgeri_, No. 6865 (KU), 2 mi. up Mink Creek, Pocatella, Bannock Co., Idaho. Left upper (I), DP4-M3; left lower (J), dp4-m3. K and L. _Thomomys talpoides fossor_, No. 13205 (KU), Wasson Ranch, 3 mi. E Creede, Mineral Co., Colorado. Right lower (K), p4-m3; left upper (L), P4-M3. ] Relationship of the Geomyini with the ancestral Dikkomyini is most clearly demonstrated in the sequence of wear on the occlusal surfaces of the molars. As in all geomyids, the upper part of the crown is biprismatic in the newly erupted tooth, and the two columns are separated by an intervening valley. With slight attrition on the unworn enamel cap, the weakly developed cusps merge and form a transverse enamel loop on each of the two columns (see third molar in Fig. 5, A and E), each loop enclosing a core of dentine that had become exposed. The valley between the two columns is shallow, and upon further wear of the tooth, the two loops unite. The two columns become joined at different points in the upper and lower molars depending on the varying depth of the valley in different teeth. Therefore, upper and lower molars develop distinctly different occlusal configurations.