Skull.—Of medium size; rostrum relatively short; nasals with posterior borders rounded; postorbital ridges extending across anterior fourth of parietals; zygomatic arches moderately strong; postorbital process of zygoma involving both jugal and squamosal; incisive foramen of medium length (4 to 5 mm) and narrow (about 2.5 mm), oval and extending caudad to a plane approximately 2 mm anterior to premolars; vomerine sheath complete, with premaxillae forming approximately anterior 3/4 of sheath; maxillary part of sheath short but well-developed; mesopterygoid fossa extending forward as far as centers of third molars; bullae of medium size but well-inflated.
Teeth.—Upper molariform teeth with three counterfolds. Lower molariform teeth with three counterfolds, except third molar which has only two.
Comparison.—From P. g. oris, leioprimna differs in: conspicuously wider aristiforms; shorter and narrower incisive foramen; lower molariform teeth with three counterfolds (except m3 with only two), instead of lower molars with only two counterfolds. Differences from P. g. nesiotes are given in the account of that subspecies.
Remarks.—The paratype was collected in an "igarapé," depression usually invaded by the river waters; the paratype, collected on 18 January, 1911, had two large embryos.
Specimens examined.—Total number, 4 (2 CNHM, 2 AMNH), from Brazil, Pará, Cametá.
Proechimys guyannensis oris [Thomas]
Proechimys oris [Thomas], September, 1904, Ann. Mag. Nat. Hist., 14 (ser. 7): 195; [Thomas], 1905, Ann. Mag. Nat. Hist., 15 (ser. 7):587; [Thomas], 1912, Ann. Mag. Nat. Hist., 9 (ser. 8):89; [Tate], 1935, Bull. Amer. Mus. Nat. Hist., 68:400; [Osgood], 1944, Zool. Ser. Field Mus. Nat. Hist., 29:199.
Proechimys cayennensis oris [Ellerman], 1940, The families and genera of living rodents, Brit. Mus. (Nat. Hist.), 1:121.
Type locality.—Igarapé-assú, E. F. B., near Belem, Igarapé-assú, Pará, Brazil. Type: British Museum (Nat. Hist.), no. 4.7.4.78, old male; collected on 6 March, 1904, by Alphonse Robert; original number, 1818.