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.