_General characters._--Size medium to large; tail short; aristiforms long and narrow; general color on upper parts Ochraceous-Buff to Ochraceous-Orange, finely and uniformly lined with blackish and not forming evident dark band on back; feet dorsally white or gray; underparts of body and inner sides of legs white; skull elongate and slender with moderate ridges; incisive foramen of medium size; vomerine sheath complete or incomplete; bullae large and elongate; upper molariform teeth with three counterfolds; lower molariform teeth with three counterfolds but commonly one or two molars have only two although premolar may have four. _Remarks._--The identity of "_Echimys longicaudatus_ Rengger" can be ascertained only after samples have been collected in the area indicated by Rengger: "unter dem ein und zwansigsten Breitengrade" in Paraguay. Of the samples available to me, those from Urucum, in western Brazil, are geographically nearest the type locality. North of Urucum, both in Brazil and Bolivia, two species of _Proechimys_ live together and one of them is the same species as that at Urucum. Of the two species found to the northward in Brazil and Bolivia, the one that ranges farther south probably will occur at the locality indicated by Rengger. Provisionally, therefore, the name _longicaudatus_ is allocated to the Urucum sample (see Osgood, 1944:198). In fact, the lack of a type specimen and the general nature of Rengger's description make "_Echimys longicaudatus_" a _nomen vanum_. If two species are found living together in the region of northern Paraguay indicated by Rengger it probably will be impossible to be sure to which one his vague description applies. The form from Urucum, to which the name _Proechimys longicaudatus_ is here applied, is undoubtedly closely related to _Proechimys leucomystax_ Ribeiro, from Utiarití, on the Rio Papagaio and also to _P. roberti_ and _P. boimensis_, all from Brazil. _P. longicaudatus_ is used as the name of the species because it is the oldest of the four names. [Illustration: FIGS. 45, 48. _Proechimys longicaudatus boimensis_, male, MCZ no. 30881, Boim. × 1.] [Illustration: FIGS. 46, 49. _Proechimys longicaudatus longicaudatus_, male, AMNH no. 37085, Urucum. × 1.] [Illustration: FIGS. 47, 50. _Proechimys longicaudatus leucomystax_, male, AMNH no. 37509, Tapirapoã. × 1.] [Illustration: FIGS. 51, 52. _Proechimys longicaudatus roberti_, male, MN no. 6233, Pouso Alto, Goiaz. × 1.] [Illustration: FIGS. 53, 54. _Proechimys longicaudatus boimensis_, male, MCZ no. 30881, Boim. × 1.] [Illustration: FIGS. 55, 56. _Proechimys longicaudatus longicaudatus_, male, AMNH no. 37085, Urucum. × 1.] [Illustration: FIGS. 57, 58. _Proechimys longicaudatus leucomystax_, male, AMNH no. 37509, Tapirapoã. × 1.] [Illustration: FIGS. 59, 60. _Proechimys longicaudatus roberti_, male, MN no. 6233, Pouso Alto, Goiaz. × 1.]