_P. setosus_ is poorly represented, both of the available skins being faded. Furthermore, no type locality is known for the subspecies _P. s. setosus_. _P. albispinus_ has only two known subspecies: _P. a. albispinus_, living in a region of higher humidity, is slightly the darker and has subapical zones of the setiforms on the sides Ochraceous-Tawny; _P. a. sertonius_, living in a much drier region, has the same subapical zone Ochraceous-Buff. The number of specimens of _P. a. sertonius_ is so few that no gradient can be detected, even if one exists. TAXONOMIC CHARACTERS 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).