The mandible is relatively long. Its masseteric ridge is well developed and massive. The basitemporal fossa is usually deep and well defined; it tends to be shallow in the subgenus Orthogeomys, and in young individuals is hardly more than a slight depression.
Key to the Subgenera of Orthogeomys
A Frontal wide and greatly inflated; no interorbital constriction; enamel plate on posterior wall of P4 usually absent, although sometimes having small plate, restricted to lingual end of wall. Subgenus Orthogeomys
A´ Frontal narrow and not greatly inflated; interorbital region decidedly constricted; enamel plate on posterior wall of P4 always present but short and restricted to lingual end of wall.
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
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