_Skull._--Strong; supraorbital ridges raised and extending across anterior fourth of parietals; nasals elongate; bullae rounded, inflated, with shallow grooves; postorbital process of zygoma weakly developed and constructed entirely of jugal; incisive foramen elongate and narrow (5.5 x 2.5 mm), posteriorly constricted with posterior margins elevated above surface of bones; vomerine sheath incomplete or complete but, when complete, with maxillary part filiform and delicate; mesopterygoid fossa extending forward as far as middle, or even anterior, parts of third molars; posterior palatine foramina on plane of centers, or even anterior faces, of second molars. _Teeth._--Upper molariform teeth with three counterfolds. Lower molariform teeth: premolar with four counterfolds, molars with three counterfolds, sometimes with two folds coalesced in center of tooth. _Comparison._--From _P. g. ribeiroi_, and _P. g. bolivianus_, _villicauda_ differs in wider aristiforms. From _P. g. ribeiroi_, _villicauda_ further differs in: larger and wider incisive foramen; vomerine sheath incomplete or complete instead of always complete and thick. From _P. g. bolivianus_, _villicauda_ differs in: lower premolars always, instead of rarely, with four counterfolds, and m3 always with three counterfolds instead of usually with only two counterfolds. _Specimens examined._--Total number, 4, from Brazil, Mato Grosso, as follows: _Cáceres_, Tapirapoã, 3 (MN); _Diamantino_, Rio Papagaio, Utiarití, 1 (AMNH). [Illustration: FIG. 62. _Proechimys guyannensis villicauda_, male, MN no. 1932, Tapirapoã. Type. × 1.] [Illustration: FIG. 63. _Proechimys guyannensis ribeiroi_, male, MN no. 1935, Rio Doze de Outubro. Type. × 1.] [Illustration: FIG. 64. _Proechimys guyannensis hyleae_, male, MCZ no. 30887, Tauarí. Type. × 1.] [Illustration: FIG. 65. _Proechimys guyannensis nesiotes_, male, CNHM no. 19496, Ilha de Manapirí. Type. × 1.] [Illustration: FIG. 66. _Proechimys guyannensis leioprimna_, female, CNHM no. 19503, Cametá. Type. × 1.] [Illustration: FIG. 67. _Proechimys guyannensis oris_, male, CNHM no. 19495, Providencia. × 1.] [Illustration: FIG. 68. _Proechimys guyannensis arescens_, male, CNHM no. 26440, Fazenda Inhuma. Paratype. × 1.] [Illustration: FIG. 69. _Proechimys guyannensis riparum_, female, AMNH no. 143018, Manaus. Type. × 1.] [Illustration: FIG. 70. _Proechimys guyannensis arabupu_, male, AMNH no. 75816, Arabupu. Type. × 1.] [Illustration: FIG. 71. _Proechimys guyannensis villicauda_, male, MN no. 1932, Tapirapoã. Type. × 1.] [Illustration: FIG. 72. _Proechimys guyannensis ribeiroi_, male, MN no. 1935, Rio Doze de Outubro. Type. × 1.] [Illustration: FIG. 73. _Proechimys guyannensis hyleae_, male, MCZ no. 30887, Tauarí. Type. × 1.]