206 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. X. 



and forming obtuse angles in front and behind; audital bullae small; 

 teeth small. As compared with a skull of O. eliurus from Sapucay, 

 Paraguay, that of 0. chaparensis is slightly larger and more angular; 

 the interorbital region is a little wider, higher, and more sharply edged; 

 nasals broader; palatine foramina shorter. 



Measurements. Type: Total length 215; head and body 104; tail 

 in; hind foot 26; ear from notch (dry) 14. Skull of type: Greatest 

 length 26.8; basilar length 21 ; zygomatic breadth 13.5; breadth of brain- 

 case 11.2; interorbital constriction 4; nasals 10.5x3.5; palatine foramina 

 4.4x1.9; diastema 6.9; length of zygomatic plate 2.7; upper toothrow 3.6. 



Remarks. Although the state of knowledge of the genus Oryzomys 

 makes it difficult or practically impossible to determine the exact 

 relationship of isolated species, it is apparent that this one does not show 

 many similarities to any of the recently described Peruvian and Bolivian 

 species and it is therefore probable that its allies are among the species 

 described by early authors from eastern South America. Of those 

 available for comparison, 0. eliurus is nearest in size but its coloration 

 is of the usual type and its skull differs in various ways. The coloration 

 of the under parts in O. chaparensis is characteristic, for although the 

 differentiation of self-colored light areas on the pectoral and inguinal 

 regions is not unusual among rodents, it is relatively rare in the genus 

 Oryzomys. 



Only two specimens have been examined, an adult male and a female. 

 This female was nursing young and has the mammae functionally 

 enlarged and conspicuous but shows no trace of more than three pairs 

 whereas four pairs are normal in Oryzomys. The absence of the ante- 

 rior pair of pectoral mammae may be accidental in this specimen or 

 it may be a peculiarity of the species. 



(Ecomys mamorae Thomas. 



Ten specimens, Todos Santos, Chapare' River. 



Since their external and cranial measurements agree closely with 

 those of the published description of this species, there is scarcely any 

 reason to doubt that these specimens are practically typical represen- 

 tatives of it and so far as recorded the only ones extant except the type 

 which is preserved in alcohol and therefore not trustworthy for color 

 characters. The color, especially that of the under parts, as shown by 

 the present series, is somewhat variable. In the majority, particularly 

 those not fully mature, the rich, tawny ochraceous lateral line which 

 borders the color of the upper parts widens and extends with only slight 

 dilution across the middle of the belly and forward as a light wash to 

 the midpectoral region. Across the middle of the belly and breast 



