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_.
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.
Animals with narrow aristiforms tend to have a more or less uniform
coloration throughout the dorsal parts. The blackish distal parts of the
aristiforms regularly interline the ground color made by the subapical
zone of the setiforms. If, on the contrary, wide aristiforms occur, the
dorsal surface is conspicuously marked by the wide blackish lines among
spots of color formed by the subapical zones of the setiforms. No clinal
variation was detected in width of aristiforms but geographic variation
in width was noted; for example, the subspecies of _P. iheringi_ differ
in this respect.
The setiforms are narrow and flattened but are without pronouncedly
raised margins. The setiforms are usually bicolored on the dorsal and
lateral surfaces of the animals, with a subapical zone of some
reddish-brown color, like Ochraceous-Orange or Ochraceous-Buff. They are
whitish or gray on the basal parts and gradually blacken toward the tip,
but have a reddish subapical zone. Common exceptions to this pattern are
setiforms without subapical zones; these appear on the dorsal surface
among setiforms which are normal in possessing distinctive subapical
zones. Also there are setiforms without blackened tips on the lateral
surfaces. Due to their relative abundance and subapical color, these
setiforms are responsible for the dominant color on the upper parts.
Like the aristiforms, they are longer and wider in the middorsal region
of the animal and are gradually less developed on the remainder of the
upper parts. Actually there is more than one type of setiform in the
hair constellation; they vary in length, width and color. Attention was
not given, however, to every type of setiform.
The ventral surface of the body and the inner sides of the legs are
uniformly covered by short setiforms, thinner and more sparsely
distributed on the inner side of the legs. These setiforms are usually
uniformly white in color or, sometimes, the distal parts are buff or
more richly colored.
Vibrissiforms are scattered on the dorsal and lateral surfaces of the
body, and in penicillate arrangements on the head. They are longer than
the pelage proper, have a nearly circular cross-section and are blackish
in color.