_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 Zoology 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., Genéve, 9:145; Waterhouse, 1848, Nat. Hist. Mammalia, 2:334. _Proechimys longicaudatus_ Thomas, 1901, Ann. Mag. Nat. Hist., 8 (ser. 7):532; Thomas, 1904, Proc. Zool. Soc. London, p. 240; Allen, 1916, Bull. Amer. Mus. Nat. Hist., 35(30):569; Tate, 1935, Bull. Amer. Mus. Nat. Hist., 68(5):400.