_Genotype._--_Echimys trinitatis_ Allen and Chapman, by original
designation.
_Proechimys_ Allen and Chapman, 26 December 1899, Bull. Amer. Mus.
Nat. Hist., 12(20):264, orig. description; Tate, 1935, Bull. Amer.
Mus. Nat. Hist., 68(5):398; Ellerman, 1940, The families and genera
of living rodents, Brit. Mus. (Nat. Hist.), 1:115.
_General characters._--Muriform echimyids of medium size; pelage
with flattened and lanceolate and sometimes clavate aristiforms,
varying greatly in width and distributed over most of the dorsal
surface from shoulders to hips or base of tail; setiforms also
flattened, evenly distributed throughout; entire ventral surface
and inner sides of legs white or, rarely, invaded by some buffy
color; tail shorter than, equal to, or slightly longer than, head
and body, bicolored and with a few bristles between scales,
sometimes heavily penicillated; feet long and narrow; pollex
rudimentary; hallux shorter than fifth toe; ears wide and long;
mammae 2-1=6.
_Skull._--Generally elongate and strongly built, with supraorbital
ridges well developed, frequently extending across parietals
toward occipital region; zygomatic arches variable in depth,
always with postorbital process; infraorbital foramen with or
without lower groove for transmission of nerve; incisive foramen
usually large; vomerine sheath complete or incomplete;
mesopterygoid fossa extending forward at least to plane of third
molars; bullae large; angular process of mandible turned upward.
[Illustration: FIGS. 18-21. Occlusal views of the upper left and lower
right molariform teeth of the two subgenera of the genus _Proechimys_.
Anterior end of the tooth row at the top of drawing. All × 6.
FIGS. 18-19. _Proechimys_ (_Proechimys_) _goeldii steerei_, sex ?,
USNM no. 105537, "Hyutanaham." Upper teeth at left (fig. 18).
FIGS. 20-21. _Proechimys_ (_Trinomys_) _dimidiatus_, male, MN no.
6256, Pedra Branca. ]
_Teeth._--Incisors opisthodont, orthodont or proodont, not grooved;
upper molariform teeth with a main internal fold and one to five
external counterfolds which usually appear as enamel islands in
worn teeth, these counterfolds barely implicating the lateral wall;
lower molariform teeth with folds as in the upper molariform teeth
except that they are reversed and the number of internal
counterfolds is usually fewer in the molars.