Size and Proportions of External Parts
Absolute size of head and body, tail, hind-foot and ear are useful in distinguishing subgenera and subspecies and to some extent in differentiating species.
The length of head and body is large to medium in Proechimys and medium to small in Trinomys. The tail is long to medium in Trinomys and short in Proechimys. The longest tail, 242 mm, is found in P. i. denigratus, and the shortest tail, 123 mm, in P. g. steerei. The relative length of tail also provides gradients or clines.
In every species, males surpass females in average size. Nevertheless, the largest animals are usually females. How this paradoxal fact is to be accounted for, I am not sure, but it may be that the animals grow as long as they live and that females have more chances to survive longer since the care of the young keeps them closer to shelter.
Color.—Upper parts vary from Buckthorn Brown to Ochraceous-Buff. Dark color ordinarily is correlated with an environment of higher degree of humidity and light color with lower humidity. However, species may be found in similar conditions of humidity but differing in color. Proechimys albispinus albispinus, for example, a light-colored form, is found in areas where the rainfall averages 1,000 to 1,500 mm of annual precipitation, in the isohygra of 80 per cent relative humidity. These conditions actually are similar to those where P. dimidiatus, of darker color, is found. The subspecies albispinus, however, ranges mostly over a dry area and the fact that it occurs also in a moist area without appreciable change in color is difficult to explain.
Insular populations are usually darker or richer in color than corresponding continental populations. On a small island, uniformity of environment and inbreeding may be responsible for an accumulation of characters for richness of color.
Pelage
The pelage provides most useful taxonomic characters. Excepting the vibrissiform hairs, all of the elements of the pelage have a common feature, the flattened shape. The hair constellation (cf. Toldt, 1935) on the upper and lateral surfaces is composed of hairs of two main types: aristiforms (guard hairs) and setiforms (over hairs).
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.