522 



PROCEEDINGS OF THE NATIONAL MUSEUM 



Some species of the arboreal genus Rhipidomys resemble subgenus 

 Oecomys in superficial characters. Both groups have evolved as 

 arboreal forms along parallel lines but Rhipidomys is more special- 

 ized. Its tail (fig. 56) is more hairy and, in certain species, considerably 

 longer; the fifth hind toe is longer and a brown metatarsal patch is 

 generally present. The very closely related Nyctomys (fig. 5a) is 

 another arboreal cricetine that resembles subgenus Oecomys in external 

 characters. Rhipidomys and Nyctomys are members of the pero- 

 myscine group of cricetines which differs from oryzomyines chiefly by 

 its short palate (pi. 2d) without pitted posterolateral palatal depres- 

 sions, some minor dental characters (pi. 4), and by the mammary 

 formula, 1—2=6. 



Additional comparisons of subgenus Oecomys are given under the 

 individual species headings. 



Some Arboreal Adaptations 



Hind foot (fig. 4) : The pes of subgenus Oecomys is actually and 

 relatively shorter and broader than that of closely related terrestrial 

 species of comparable body size. Measurements of the length of the 

 hind foot of a series of the small species Oryzomys (Oecomys) bicolor 

 are shown in table 1. Thej may be compared with measurements in 



Explanation of Symbols in Figure 3 



Names of Cusps 

 Upper Molars 



1, Protocone 



2, Hypocone 



3, Paracone 



4, Metacone 



5, Mesoloph 



6, Mesostyl 

 7-12, Procingulum 



7, Anteroloph 



8, Anterolabial style 



9, Anteroconule 



10, Anterolingual style 



11, Anterolabial 



conule 



12, Anterolingual 



conule 



13, Posteroloph (post- 



cingulum) 



14, Enterostyle 



15, Paralophule 

 Lower Molars 



1, Protoconid 



2, Hypoconid 



3, Metaconid 



4, Entoconid 



5, Mesolophid 



6, Mesostylid 

 7-12, Procingulum 



7, Anterolophid 



8, Anterolingual stylid 



(not shown) 



9, Anteroconulid 



10, Anterolabial stylid 



11, Anterolingual 



conulid 



12, Anterolabial 



conulid 



13, Posterolophid (post- 



cingulum) 



14, Ectostylid 



15, Ectolophid 



16, Entolophulid 

 Names of Folds 



mf, major fold 

 nf 1, first minor fold 

 pf 1, first primary fold 

 pf 2, second primary fold 



sf 1, first secondary fold 

 sf 2, second secondary 



fold 

 if 1, first internal fold 



(shown coalesced 



with pf 1) 

 if 2, second internal fold 



(shown coalesced 



with pf 2) 

 aif, anterior internal fold 

 asf, anterior secondary 



fold (upper first 



molar only) 

 alf, anterior lingual fold 



(upper first molar 



only) 

 abf, anterior labial fold 

 (lower first molar 



only) 

 apf, anterior primary 

 fold (lower first 

 molar only) 



