12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. DO 



Isthmian representative of the genus seems to be a distinct species 

 as shown by comparison with specimens of hydrochceris from Suri- 

 nam, Brazil and Paraguay. In Panama it is apparently restricted to 

 a limited area near the head of tide-water in the delta region of the 

 Tuyra and Chucunaque rivers. 



Specimens examined. — Six, all from the type locality. 



ISOTHRIX DARLINGI, 1 sp. nov. 



Type from Marraganti (near Real de Santa Maria), on the Rio 

 Tuyra, eastern Panama. No. 179577, female young, U. S. National 

 Museum (Biological Survey Collection), collected by E. A. Gold- 

 man, May 11, 1912. Original number 21739. 



General characters. — Size rather small ; color pale ; head grayish 

 brown ; ears tufted ; hind feet short and broad, armed with short, 

 sharp, strongly curved claws ; tail unicolor, evenly tapering, thinly 

 clothed with short hairs, becoming a trifle longer but not thicker or 

 forming a distinct brush near tip. Somewhat similar to labilis, but 

 smaller and much paler in color ; differing also in cranial characters. 



Color. — Upper parts ochraceous buff, somewhat darkened by 

 admixture of black-tipped hairs along back, becoming grayish brown 

 on middle of face and cheeks ; nose, lips and muzzle mouse gray, 

 this color abruptly interrupted at posterior base of whiskers by a 

 white vertical streak ; orbital rings black ; a small white supraorbital 

 spot ; ears brownish or blackish, with a marginal fringe of longer 

 hairs, and tufts of the same length and color near anterior and poste- 

 rior base ; inner base of ears clothed with a partially concealed patch 

 of whitish fur ; whiskers and a postorbital tuft of long vibrissas black ; 

 under parts ochraceous buff, becoming grayish on wrists, ankles 

 and chin ; feet silvery white ; tail brownish. 



Skull. — Similar to that of labilis, but smaller and relatively 

 broader ; zygomata more spreading anteriorly ; audital bullae larger, 

 more inflated ; dentition about as in labilis. In general form and 

 dentition much as in Isothrix caniceps as figured by Giinther, but 

 palate narrower and premaxillae reaching posteriorly beyond nasals. 



Measurements. — Type: Total length, 472 mm. ; tail vertebrce, 240 ; 

 hind foot, 44. Skull (type) : Greatest length, 53.5 ; condylobasal 

 length, 49.5; zygomatic breadth, 25.2; nasals, 14.4; interorbital 

 breadth, 12.3; palatal length, 23.1; length of palatal bridge, 12.8; 

 maxillary tooth row, 12.9. 



1 Named for Dr. S. T. Darling, of the Sanitary Department, Isthmian Canal 

 Commission, who first collected the species, at Ancon, Canal Zone. 



