Skull
The absolute size of the skull is proportionate to bulk of the body. The supraorbital and parietal ridges are especially developed in the P. semispinosus group, where they extend across the parietals to the interparietals. In all members of the subgenus Proechimys, these ridges extend onto the parietal region. In Trinomys, however, they do not extend so far posteriorly as the parietal, but only onto the squamosal.
The rostrum varies from slender to stout. Elongate rostra are common in Proechimys; Trinomys has a short blunt rostrum.
The infraorbital foramen commonly has a ventral groove for nerve transmission in many forms of Proechimys but Trinomys almost always lacks this groove. Presence or absence of the groove is a subspecific character in the subgenus Proechimys.
The jugals are dorso-ventrally wide in Trinomys except in the species P. setosus. In Proechimys a dorso-ventrally narrow jugal is the rule, but P. canicollis has an especially wide jugal. A postorbital process appears on the jugo-squamosal suture and is here called postorbital process of the zygoma. In Proechimys it is more or less weakly developed and shows no variation of systematic worth. In Trinomys, on the other hand, this process varies in a clinal way (P. iheringi) and stages of the gradient characterize populations of subspecific rank.
Linear and spatulate shape of the humular process of the pterygoid constituted specific characters for [Thomas], but there is so much individual variation in the shape of this process in almost every population that it has not been used in the present account.
The mesopterygoid (interpterygoid) fossa in almost every specimen extends anteriorly to the level of M1 or M2 in Trinomys, and to M3 in Proechimys. Exceptions may occur, as in P. hendeei, where the fossa extends to the level of M2.