230 MEMOIRS OF THE CARNEGIE MUSEUM 



Head 3.66 in the length, measured to end of opercle and end of the lateral 

 plates; depth 6; D. 1,7; A. 1,4; V. 1,5; P. 1,6; lateral line 24; eye 5.5 in snout, 7.5 

 in head to end of opercle, 3 in interorbital. 



Oral disk everywhere thickly papillose, the papillae largest along the margin of 

 the upper lip and smallest at the angle of the mouth; barbel about 2.5 in the snout, 

 its free portion less than orbit in length, its ventral surface papillose; maxillary 

 and dentary of about equal length, a little less than 1.5 in the interorbital, each 

 jaw with numerous minute teeth. 



Ventral surface entirely naked; margin of snout in front of base of barbels 

 naked; predorsal scales not regular; posterior margin of skull concave on each side 

 of the occipital crest, which is indicated by a point; no ridges or grooves about the 

 head; lateral plates straight, not keeled; a rather broad naked area along the dorsal. 



Origin of dorsal in front of the vertical from the ventrals; ventrals a little in 

 advance of the middle; highest dorsal ray reaching to about the middle of the last 

 ray, 4.5 in the length; caudal emarginate, the lower lobe slightly the longer; origin 

 of anal under vertical from middle of last dorsal ray; ventrals reaching middle of 

 anal, pectorals past origin of ventrals. 



Dark, with faint lighter spots; fin-rays dark, the membranes hyaline. 



This species, obtained at my farthest point, is named for Mr. C. Wilgress 

 Anderson, Government Surveyor, an explorer in the Potaro and Roraima regions. 



Hemiancistrus Bleeker. 

 Hemiancistrus Bleeker, Nederl. Tijdschr. Dierk., 1,1863,78 (medians) . — Eigen- 



mann and Eigenmann, Occasional Papers Cal. Acad. Sci., I, 1890, 417. 



Type, Ancistrus medians Kner. 



Teeth small; snout granular to its margin, without bristles; interopercle with 

 spines; an adipose fin. 



A genus of about a dozen species, ranging from the Pacific slope of Panama to 

 eastern Peru and Paraguay. 



Three species seem to occur in British Guiana, although only one was actually 

 secured. They may be distinguished as follows: 



Key to the Guiana Species of Hemiancistrus. 

 a. Interorbital 2.66 in the head; depth S in the length; head 2.6 times as long as deep; mandibular 



ramus 2.33 in interorbital schomburgki. 



an. Interorbital 3.5 in the head; depth 4-4.5 in the length; head 1.66-1.75 times as long as deep; man- 

 dibular ramus l.S in the interorbital; lower surface naked megacephalus. 



aaa. Interorbital 2 in the head; depth 4.75 in the length; mandibular ramus 2 in the interorbital; 

 occipital bluntly keeled; lower surface partly covered braueri. 



