102 Proceedings of the Biological Society of Washington. 



vertically ; palate longer, reaching posteriorly beyond the posterior plane 

 of last molars, the greater extension due to expansion of the palatines; 

 audital bultemuch flattened and angular instead of rounded and inflated, 

 the outer edges overlapped by extensions of the alisphenoids and squa- 

 mosals, molariform teeth smaller; upper premolars less extended antero- 

 posteriorly, the anterior of the series more distinctly smaller than the 

 second. Contrasted with a skull without definite locality, but believed to 

 be from tlie Lower Amazon or Guiana and assumed to represent A. ater, 

 the frontal region is similarly prominent and the palate as a whole is of 

 about the same length, but the palatine portion of the bony slielf is 

 longer, reaching anteriorly to the posterior plane of first molars; the 

 zygomatic portion of the jugal is much heavier, more expanded vertically, 

 and the audital bullae much smaller, less inflated. 



Measurements. — Type: Total length, 1260; tail vertebrae, 810; hind 

 foot, 170. Skull (type): Greatest length, 113.9; occipitonasal length, 

 98.3; basal length, 79; breadth of braincase, 59; zygomatic breadth, 

 65.7; orbital breadth, 57.3; postorbital breadth, 44.7; breadth of rostrum 

 at canines, 26.2; greatest width of nasals anteriorly, 10.4 ; palatal length, 

 35.5 ; upper molar series, 22; lower molar series, 27. 



Remarks. — The Darien representative of the A. ater group is externally 

 distinguished from its geographic neighbor, A. robustus of Colombia, by 

 the proportionately longer tail ; the skull may be known by the posterior 

 extension of the palate beyond the last molars. Its exact relationship to 

 A. ater of Guiana is somewhat problematical owing to unsatisfactory 

 material for comparison, but the latter appears to be a shorter-tailed 

 animal with cranial differences already pointed out. Moreover, the 

 cranial measurements given by Elliot* indicate that J., ater in a larger 

 animal. Ateles rufiventris Sclater, which was described from the Rio 

 Atrato and may range north to Panama, seems amply distinguished by 

 its color. 



Specimens examined : One, the type. 



■ Rev. Primates, vol. 2, p. 30, 1913. 



