ARBOREAL RICE RATS — HERSHKOVITZ 543 



tatives of concolor are hardly larger than some large southern individ- 

 uals of 0. bicolor (pi. 5). In the Guianas, size alone is not a decisive 

 character. Generally, the heavier more "adult" skull of concolor 

 with its better developed supraorbital and temporal ridges, less inflated 

 braincase, and more gray or buff underparts are characters that help 

 separate the larger from the smaller species where the two occur 

 together. 



Representatives of Oryzomys suhflavus Wagner^ of Brazil and the 

 Guianas have sometimes been confused with 0. concolor. The two 

 species are similar in color, size, and tail length. In suhjlavus, however, 

 the pelage is comparatively lax and harsh; hind foot (fig. 4c) long, 

 stout, with outer digits shorter, first hind toe with claw barely ex- 

 tending beyond base of second toe, fifth hind toe with claw extending 

 only to base of middle phalanx of fourth toe; sole of heel exposed; 

 excision of dorsal border of antorbital foramen deep, one-half or more 

 of width of anterior zygomatic plate exposed to view when skull is 

 viewed from above; supraorbital region of skull (c of pis. 8-11) broad, 

 its sides ridged or beaded and divergent as in concolor; zygomata less 

 expanded anteriorly; incisive foramina generally narrower but with 

 their posterior borders not extending behind anterior plane of first 

 molars ; sphenopalatine vacuities more open but less so than in 0. 

 xanthaeolus (compared below); bullae more inflated; enamel pattern 

 of molars (pi. 12c) essentially as in Oecomys. 



Oryzomys xanthaeolus Thomas is a semiarboreal species with a 

 superficial resemblance to 0. concolor. It occupies the dry coastal 

 area of western Peru and southwestern Ecuador where Oecomys is not 

 known to occur. Its pelage is comparatively lax, upperparts and 

 sides of body buffy or olivaceous, lateral line absent, underparts 

 not pure white; tail long as in concolor but un tufted and bicolor for 

 most or all its length; hind foot broader than in strictly terrestrial 

 species but narrower than in concolor, the outer toes shorter and less 

 robust, interdigital webbing more developed; general shape of skull 

 {d of pis. 8-11) and sides of supraorbital region as in concolor but 

 anterior zygomatic plate projected forward so that at least one-half 

 its width is visible from above; incisive foramina longer, their length 

 at least three-fourths that of diastema, and terminating posteriorly 

 well behind anterior plane of first molars; sphenopalatine vacuities 

 large, as in 0. palustris (pi. 2c), posterior border of palate without 

 excrescences; bullae well inflated; enterostyles and ectostylids (pi. \2d) 

 rudimentary or absent. 



» "Oecomys" catherinae Thomas is a synonym of Oryzomys stibflaims. "Oecomys" rex Thomas, 1910, of 

 British Quiana, is also a member of the same species but probably subspeclfically distinct. The name, 

 however, clashes with "Calomys" ["Oryzomys] rex Wlnge, 1888. Because of the homonymy, the name of 

 the form described by Thomas may be changed to Oryzomys subflavus regalis (new name). 



