ARBOREAL RICE RATS — HERSHKOVITZ 521 



quently in M^; first minor fold reduced or obsolete in unworn M^, 

 obsolete or absent in M^; anterior border of procingulum of unworn 

 first upper molar sometimes notched; mesostyle always fused with 

 mesoloph and connected with either paracone or metacone, or both; 

 a well-developed paralophule usually present and often extending 

 across first secondary fold to connect with mesoloph; primary folds 

 often irregular in outline, the second primary usually isolated from 

 the margin; first and second internal folds of M^"^ discrete or confluent 

 with corresponding primary folds, of M^ discrete or confluent with 

 primary or secondary folds; first secondary fold present in all upper 

 molars, usually as an enamel island or as two, sometimes three, is- 

 lands separated by the paralophule; second secondary fold present in 

 M^"^ as an enamel island, in M^ as a discrete island, or may be coalesced 

 with either second primary or second internal fold, or absent. Pro- 

 cingulum of Ml subovate in outline, never distinctly bilobate, the 

 internal, labial and lingual folds isolated from the margin, discrete 

 or coalesced with each other; apex of major fold of Mi. 2 hardly or not 

 at all extending beyond midline of tooth; first minor fold well de- 

 veloped in all lower molars; ectostylid well developed in all molars; 

 ectolophid always present, frequently well developed and fused with 

 ectostylid; a well-defi.ned fold between hypoconid and ectolophid 

 often present and isolated from the margin; first and second internal 

 folds coalesced with corresponding primary folds; first secondary fold 

 of M2.3 obsolete or absent; second secondary fold of Mi.2well developed, 

 usually isolated but clearly defining mesolophid, of M3 obsolete or 

 absent; a small entolophulid sometimes present in one or more molars. 



Comparisons 



Subgenus Oecomys is the oryzomyine most specialized for arboreal 

 life. Its adaptive characters contrast sharply with those of true 

 Oryzomys as represented by its typical species, the palustrine, riparian 

 0. palustris Harlan. In palustris, pelage of upperparts is dark and 

 glossy, middle digits of hind feet (fig. 4/) long, the outer ones short; 

 interdigital webbing nearly as well developed as in the aquatic 

 oryzomyine Nectomys; fifth postdigital plantar tubercle reduced or 

 lost as in many aquatic Muridae; tail coarsely scutulated and often 

 provided with keel haii"s. The skull of 0. palustris (pis. 1-4) differs 

 from subgenus Oecomys chiefly by the prominence of the forward- 

 projecting zygomatic plate when viewed from above and by the ex- 

 tremely large sphenopalatine vacuities. The molars of palustris and 

 subgenus Oceomys (pi. 4) are similar but the lower internal folds are 

 usually discrete in those of palustris. Except for the presence of 

 intermediate forms, typical Oryzomys and subgenus Oceomys might 

 well be treated as generically_^distinct. 



