This is one of the more specialized genera of the Geomyini. Save for one record in the late Pleistocene (_Orthogeomys onerosus_), there is no fossil history of the genus upon which to reconstruct its phylogeny; therefore, its phyletic development must be estimated by comparing it and the primitive morphotype of the tribe. Results of that comparison suggest that _Orthogeomys_ has closer affinities with _Zygogeomys_ than with any of the other genera, and that _Orthogeomys_ may have originated in an early dichotomy of primitive _Zygogeomys_ stock instead of descending from the ancestral stock of the tribe. Except for the unisulcate incisors and the longer posterior loph on the third upper molars, the teeth of the two genera do not differ significantly. As in _Zygogeomys_, the enamel blade on the posterior wall of P4 has been reduced to a short plate restricted to the lingual third of the tooth (see Fig. 7F and H). In _Orthogeomys_, the trend in reduction of enamel is carried to its extreme only in the subgenus _Orthogeomys_, where this plate has been completely lost in most taxa (see Fig. 7D). The most significant trends in _Orthogeomys_, and the principal basis for recognizing the genus, are the dolichocephalic specializations of the skull, as described elsewhere, and the adaptive traits that have equipped the genus for living in tropical environments. The dolichocephalic features are more sharply defined in the subgenera _Orthogeomys_ and _Macrogeomys_, and are less developed in the subgenus _Heterogeomys_. Aside from the general dolichocephalic specializations, trends in _Orthogeomys_ include: Increase in size; loss of the median one of the two grooves on the anterior face of the upper incisor in the ancestral stock; increase in the anteroposterior length of each of the cheek teeth, as well as the aforementioned elongation of the posterior loph of M3; compression of the lateral angles of the premolars; and the remarkable increase in the size of the rostrum. Pappogeomys