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. Incisive Foramen The shape and dimensions of the incisive foramen long have been recognized as providing specific characters. Large size of the foramen is probably correlated with the requirement for a large amount of moisture reaching Jacobson's organ in the nasopalatine space; the moistening of the sensory epithelium is certainly involved. There seems to be a certain correlation between small size of the incisive foramen and high degree of humidity in the environment. Shapes and dimensions of the foramen appear as simple or multiple biotypes and provide characters which can be employed to differentiate subspecies, species and even subgenera. Usually a character, say a general shape, occurs in nearly all populations of a given subspecies but the particular shape seems to be more closely correlated with ecological conditions, especially humidity. Animals which live far away from large rivers usually have larger foramina than animals which live close to rivers. Both the premaxilla and the maxilla develop processes which form a sheath for the vomer. This vomerine sheath forms a bridge which longitudinally crosses the incisive foramen; the structure of this bridge varies widely. Sometimes the maxillary part is not developed and the sheath is incomplete posteriorly; sometimes this maxillary part is very slender and merely touches the premaxillary part. The premaxillary part, however, is always well developed. Teeth