Skull.—Elongate and slender; rostrum slender and long; nasals evenly pointed posteriorly; supraorbital ridges weak and barely reaching anteriormost part of parietals; zygomatic arches slender; infraorbital foramen with groove for nerve-transmission obsolete; postorbital process of zygoma weak and involving mostly squamosal; incisive foramen 5 mm long and 2.5 to 3 mm wide, oval, with margins not much extended caudad as ridges; posterior margin of incisive foramen approximately 2 mm anterior to premolars; vomerine sheath complete with maxillary part slender; mesopterygoid fossa sharply pointed anteriorly and extending forward as far as anterior part of third molar; bullae of medium size, smooth and more or less triangular in contour.
Teeth.—Crown length of upper molariform teeth 7 to 7.5 mm; all upper molariform teeth with three counterfolds; lower premolar with four counterfolds; lower molars with three counterfolds each.
Comparisons.—From three of the other four subspecies found in Brazil, boimensis differs in having four, instead of three, counterfolds in the lower premolars. Differences from P. l. brevicauda are indicated in the account of that subspecies.
Remarks.—The material available from Boim, Tapajoz, is rather poor and of no great significance. The Museu Nacional has one specimen from Boim which agrees with the type in the American Museum of Natural History. The Museum of Comparative Zoölogy at Harvard College, however, has one specimen in its collection (MCZ no. 30888 [M]), adult, also from Boim, in which the color pattern is different although agreement with the type specimen is shown in cranial characters and width and length of hairs. This specimen is much darker than the other specimens (of orange-tint) assigned to this subspecies. However, in other samples of this species, similarly dark animals have been noted. It seems that the orange tint is gained only in old age. Another specimen (MCZ no. 30878 [F]) agrees in all respects with the type of the subspecies but the locality recorded on the label is Cametá, on the Tocantins River. Possibly the subspecies has a range so wide as to include Cametá but I suspect that the data on the label is incorrect as to locality. The collector marked [M] on the label although the specimen is a [F]. The mistake might have occurred through labeling of specimens at a time later than that at which they were collected. The collector was in both localities on more than one occasion.
Specimens examined.—Total number, 5, from Brazil, Pará, as follows: Santarem, Rio Tapajoz, Boim, 3 (AMNH type, MCZ 1, MN 1); Porto de Moz, Tauarí, 1 (MCZ); Cametá (?) 1 (MCZ).
Proechimys longicaudatus longicaudatus (Rengger)
Echimys longicaudatus Rengger, 1830, Naturgeschichte der Saeugethiere von Paraguay, p. 236.
Loncheres myosuros [Lichtenstein], 1832, Darstellung neuer oder wenig bekannter Säugethiere, pl. 36 and text.
Echimys myosuros Is. [Geoffroy] Saint-Hilaire, 1840, Mag. Zool., Ann. 2 (ser. 2):15, 17; [Allen], 1899, Bull. Amer. Mus. Nat. Hist., 12(20):261.
Echimys cayennensis Pictet, 1841, Mém. Soc. Phys. Hist. Nat., Genève, 9:145; Waterhouse, 1848, Nat. Hist. Mammalia, 2:334.