514 PROCEEDINGS OF THE NATIONAL MUSEUM vol. uo 



Material 



A total of 390 specimens was studied. This number includes vir- 

 tually all arboreal rice rats preserved in American museums, and the 

 types only in the British Museum (Natural History). 



The following abbreviations for museums are used in the lists of 

 specimens examined. 



AMNH = American Museum of Natural History 



BM = British Museum (Natural History) 



CM = Carnegie Museum 



CNHM = Chicago Natural History Museum 



MACN = Museo Argentine de Ciencias Naturales 



MCZ = Museum of Comparative Zoology 



UMMZ = University of Michigan Museum of Zoology 



USNM = United States National Museum 



The writer expresses thanks to the authorities of the above insti- 

 tutions for permitting him to study the material in their charge. 



The Ortzomyine Rodents 



The general characters of subgenus Oecomys are common to Oryzomys 

 and a number of other genera which together compose the oryzomyine 

 group of the subfamily Cricetinae (family Muridae). In addition 

 to the aforementioned, the group includes the currently recognized 

 genera or subgenera Melanomys Thomas, Oligoryzomys Bangs, 

 Microryzomys Thomas, Nesoryzomys Heller, Neacomys Thomas, 

 Scolomys Anthony, Nectomys Peters, Sigmodontomys J. A, Allen, 

 and Megalomys Trouessart. The group has been defined elsewhere 

 (Hershkovitz, 1944, Univ. Michigan Mus. Zool. Misc. Publ. No. 

 58, pp. 12-17). Its more salient characters may be summarized 

 as follows. 



Size and form Mus-\ike to Rattiis-like', eyes and ears normal; hind 

 feet of the generalized scansorial-terrestrial type, or moderately 

 speciahzed for running, swimming or climbing; three middle digits 

 of hind foot partly, but not always conspicuously, webbed; claws 

 short, more or less recurved, never specialized for digging; tail always 

 more than one-half as long as head and body combined, generally 

 thinly haired, the scales always visible; mammae eight (two pairs 

 pectoral, two pairs inguinal). Distance between first molars always 

 more than length of M^; median longitudinal palatal ridge not present; 

 bony palate produced posteriorly beyond plane of last molars; postero- 

 lateral palatal fossa present and marked by a distinct pit or reticula- 

 tion of pits; parapterygoid fossa shallow, its anterior corner never 

 undercut. Incisors ungrooved, generally opisthodont, never proodont; 

 molars generally brachyodont, never distinctly hypsodont; crowns 

 crested or terraced, never plane (cf., Hershkovitz, 1955, Fieldiana, 

 Zool., vol. 37, p. 649); mesoloph (id) present in all molars and fused 



