542 PROCEEDINGS OF THE NATIONAL MUSEUM vol. no 



A skin without skull from Ticunhuaya, Bolivia, not far from the 

 type locality of benevolens, appears to belong here. 



The largest specimens of 0. bicolor phaeotis are practically in- 

 distinguishable from small adults of 0. concolor from other parts 

 of tropical America. On the other hand, the size difference between 

 0. bicolor phaeotis and 0. concolor of the same general region leaves 

 no doubt regarding the separate identity of the two species. 



Specimens examined: Eleven, from the following localities: 



Peru: Sagrario, Rio Inambari, Puno, 1 (BM, type of phaeotis) ; Hacienda Villa 

 Carmen, Cosfiipata, Cuzco, 1 (CNHM); Camante, Marcapata, Cuzco, 2 (CNHM); 

 Quincemil, Marcapata, Cuzco, 1 (CNHM). 



Bolivia: Chimate, La Paz, 1 (BM, type of benevolens); El Palmal, Chapar^, 

 Cochabamba. 1 (MACN); Marbdn, Rio Mamor6, Beni, 3 (MACN); Ticunhuaya, 

 La Paz, 1 (AMNH). 



Oryzomys concolor Wagner 



Distribution (fig. 2) : Tropical and subtropical forested zones of 

 Costa Rica, Panama, Colombia, Venezuela, Trinidad, the Guianas, 

 Brazil, and the Amazonian regions of BoUvia, Peru, and Ecuador; 

 altitudinal range from near sea level to approximately 2,000 meters 

 above. 



External characters: Dorsal surface buffy to tawny or rufous, 

 with a fine to coarse mixture of dark brown; a poorly defined dark 

 middorsal band sometimes present; underparts sharply or hardly 

 defined from sides, basal portions of individual hairs white or gray; 

 tail from 49 to 60 percent of total length. 



Cranial characters (pis. 1-6, 8-11): Sides of supraorbital region 

 prominently ridged, often ledgelike; temporal ridges well developed 

 in adults for entire length of parietals; anterior zygomatic plate 

 moderately broad, its anteroposterior width at midpoint usually less, 

 rarely equal to or slightly more, than alveolar length of M^"^. upper 

 anterior corner of zygomatic plate slightly projecting; braincase 

 moderately inflated, flat or slightly convex anterodorsally at fronto- 

 parietal suture; posterior borders of incisive foramina usually termi- 

 nating anteriad to, rarely slightly behind, anterior plane of first molars; 

 posterior half of palatines often marked by a pair of small transverse 

 cornuate processes, one on each bone (one or both processes are 

 frequently lost in the cleaning operation but the median burr or 

 rugosity remains). 



Dental characters (pis. 46, 126) : Essentially as in 0. bicolor but 

 with first and second internal folds of M*"'* more frequently confluent 

 with corresponding primary folds. In extremely worn teeth, however, 

 these folds may be secondarily discrete. 



Comparisons: Oryzomys concolor is essentially an outsize 0. bicolor. 

 In most localities where the two species are sympatric the difference 

 in size is obvious. On the other hand, many small northern represen- 



