ARBOREAL RICE RATS — HERSHKOVITZ 



529 



northwestern Venezuela and in northern and western Colombia may 

 not be real. 



External characters: Dorsal surface ochraceous to tawny finely 

 mixed with dark brown; underparts sharply defined white or buffy to 

 ochraceous, the basal portions of the individual hairs white through- 

 out, or pale gray on belly and lateral borders; tail from 40-60 percent 

 of total length. 



Table 1. — Oryzomys (Oecomys) bicolor bicolor Tomes: External measurements 

 (in millim.eters) of a series of adults from the Rio Caquetd region, eastern Colombia. 

 Specimens were collected and measured in the flesh by the author. Note proportional 

 length of hind foot and compare with table 2. 



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

 finely beaded, sometimes narrowly ledgelike; temporal ridges never 

 strongly developed anteriorly, weakly defined or obsolete posteriorly; 

 anterior zygomatic plate comparativelj' narrow, its anteroposterior 

 width at midpoint always less than alveolar length of M*~^, and fre- 

 quently less than length of M^~^; upper anterior corner of zygomatic 

 plate not produced forward and barely or not at all visible when skull 

 is viewed from above; braincase well inflated, usually convex antero- 

 posteriorly at frontoparietal suture, the interparietal steeply arched; 

 position of posterior borders of incisive foramina from slightl}'- in front 

 of to shghtly behind anterior plane of first molars; posterior halves of 

 palatines without notable excrescences. 



Dental characters. Those of the subgenus. 



Comparisons: Ordinarily the difference in size between Oryzomys 

 bicolor and 0. concolor is sufficient for distinguishing the smaller from 

 the larger species. However, a few extremely large individuals of 

 0. bicolor may equal or even exceed in size small adults of some popu- 

 lations of 0. concolor. Where the similarity extends to color of 

 underparts, length of hind foot, size of molars and width of anterior 

 zygomatic plate, it may be difficult to distinguish an isolated specimen 

 of the first species from that of another of the second species. 



504676—39 2 



