The aristiforms are wide, strong, and have the dorsal (= anterior) margins raised, forming a wide shallow longitudinal groove on the dorsal face of the hair. The tip is a filament that usually is lacking in aristiforms which are especially strong. Wear probably removes these tips. The aristiforms have the bases whitish or grayish and the amount of pigment gradually increases distally to a dark brown or blackish shade. On the dorsal and lateral surfaces of the head the aristiforms are small and narrow but gradually increase in length and width caudad on the animal. The maximum development is reached in the middorsal region, from where they decrease in size and number toward the lateral surfaces or caudad. This decrease in the development of the aristiforms, however, is not uniformly gradual. Generally, the aristiforms become increasingly conspicuous in a middorsal band, but they extend to the sides and onto the outer sides of the thighs; the band narrows rapidly on the rump. In the subgenus _Trinomys_, where the aristiforms attain their maximum development, they are still strong and conspicuous on the rump and sometimes around the base of the tail. In _Proechimys_ the aristiforms do not extend caudad from the hips. Also, in _Trinomys_, besides the ordinary lanceolate type, there are some aristiforms on the dorsal surface with a clavate shape; the base is wide and the distal part narrow. This parallels the conditions in the pelage of the most spiny species in the genus _Echimys_, _Echimys chrysurus_ (Lichtenstein). The recently named subspecies _Proechimys cayennensis hoplomyoides_ Tate, 1939, shows an extraordinary development of the aristiforms on the back and sides such as occurs in the genus _Hoplomys_. Actually the small bulla, wide basisphenoid and tooth structure add to the possibility of _hoplomyoides_ being a true _Hoplomys_, and worn teeth might have been responsible for the difficulty which Tate had in allocating the form to the proper genus. However, the narrow braincase is more nearly like that of _Proechimys_ than that of _Hoplomys_. The intermediate nature of _hoplomyoides_ argues for including the genus _Hoplomys_ as a subgenus of _Proechimys_. Species with narrow aristiforms have a rather soft and flexible pelage, while those with wide aristiforms have harsh, spiny pelage. The aristiforms vary in width from 0.45 to 1.3 mm, depending upon the species or subspecies.