B Anterior margin of mesopterygoid fossa even with plane of posterior wall of M3; postorbital bar weakly developed; anteroposterior occlusal length of M3 equal to, or less than, combined length of M1 and M2. Subgenus _Heterogeomys_ p. 530 B´ Anterior margin of mesopterygoid fossa decidedly behind plane of posterior wall of M3; postorbital bar strongly developed; anteroposterior occlusal length of M3 more than combined length of M1 and M2. Subgenus _Macrogeomys_ p. 531 Subgenus =Orthogeomys= Merriam 1895. _Orthogeomys_ Merriam, N. Amer. Fauna, 8:172, January 31. _Type._--_Geomys scalops_ Thomas, 1894, from Tehuantepec, Oaxaca, México. _Chronologic range._--Known only from the Recent. _Description._--Skull elongated and narrow (many skulls of nearly uniform breadth throughout), being extreme in dolichocephalic specializations; mandibles long and narrow, rami not spreading laterally, being more nearly parallel-sided than in other subgenera; angular processes short; breadth across zygomata not significantly exceeding breadth across mastoid processes (in many skulls considerably less); interorbital area remarkably broad, lacking deep constriction; frontals between orbits greatly inflated laterally, postorbital prominence inconspicuous; mesopterygoid fossa extending to level of posterior margin of M3; I having sulcus broader than in other subgenera, mostly on inner half of anterior surface but sometimes overlapping mid-line; enamel plate lacking from posterior wall of P4, rarely retaining narrow vestige near lingual border of posterior loph; M3 having distinct heel, bicolumnar pattern with inner re-entrant fold usually minute, occlusal length less than in other subgenera, length less than combined lengths of M1-2; hair generally coarse, sometimes hispid, sparse, in lowland forms, so sparse as to impart appearance of nakedness. _Referred species and subspecies._--Fourteen taxa: _Orthogeomys grandis alleni_ Nelson and Goldman, 1930. Jour. Mamm., 11:156, May 9. Type from near Acapulco, 2000 ft., Guerrero. _Orthogeomys grandis annexus_ Nelson and Goldman, 1933. Proc. Biol. Soc. Washington, 46:195, October 26. Type from Tuxtla Gutierrez, 2600 ft., Chiapas. _Orthogeomys grandis carbo_ Goodwin, 1956. Amer. Mus. Novit., 1757:5, March 8. Type from Excurano, 2500 ft., Cerro de San Pedro, 20 km. W Mixtequilla, Oaxaca. _Orthogeomys grandis felipensis_ Nelson and Goldman, 1930. Jour. Mamm., 11:157, May 9. Type from Cerro San Felipe, 10 mi. N Oaxaca, Oaxaca. _Orthogeomys grandis huixtlae_ Villa, 1944. Anal. Inst. Biol. Univ. Nac. México, 15:319. Type from Finca Lubeca, 12 km. NE Huixtla, 850 m., Chiapas.