Pearson: Fishes of the Rio Beni Basin 
21 
equal to the head in length, the last ray V 2 as long as the first ; 
the base of the dorsal 1.25 in its distance from the adipose; the 
caudal deeply emarginate, the lobes 1.3 times longer than the 
head ; the caudal peduncle 3.2 times as long as deep, flat below. 
The head and body covered with many small dots, the dots 
on the body increasing in size posteriorly. The dorsal spotted, 
the spots arranged in a row on the interradial membrane, the 
posterior edges of the spots just touching the rays. A row 
of irregular spots on the middle of the membrane between 
each two rays. The rays of the caudal spotted, the spots 
regularly arranged in rows, pectorals and ventrals irregularly 
spotted. 
Plecostomus bolivianus Pearson, sp. nov. Plate IV, figs. 1-3. 
17011, 17, the largest the type, 34-150 mm., Popoi River, Upper Beni, 
Bolivia. Sept., 1921. 
17012, 15, 48-146 mm., Huachi, Bolivia. Aug., 1921. 
Head 3; depth 5; D.I,7 ; A. 1,4; eye 9; in the young 6; the 
length of the head only slightly longer than the width, the 
depth 1.9 times in the length, the snout 1.7 times, interorbital 
3 times; mandibular ramus 1.3 times in the interorbital space; 
supraorbital edges not raised ; supraoccipital with the median 
ridge more pronounced posteriorly; the tip of the snout with 
a large naked area; the maxillary barbel .7 the mandibular 
ramus, not reaching the gill opening by a distance equal to the 
orbital diameter; scutes spinulose, the first 4 above the pec- 
toral spine weakly carinate, 26 in a lateral series, 6 between 
the dorsal and the adipose, 13 between the anal and the caudal ; 
the supraoccipital bordered posteriorly by a single scute; the 
lower surface of the head and abdomen mostly covered with 
small spinulose plates, in the young entirely naked; the tips 
of the pectorals reaching considerably beyond the base of the 
ventrals ; ventrals reaching beyond the base of the anal, to the 
middle of the depressed anal spine; anal spine reaching pos- 
terior edge of the third plate when depressed; the caudal 
emarginate, the outer rays only slightly longer than the head ; 
dorsal spine 1.3 times in the head, the last ray just reaching 
the base of the adipose when depressed, the last ray 4/7 as 
long as the first; the caudal peduncle 2.8 times as long as 
deep, rounded below. 
The head thickly covered with small spots becoming larger 
posteriorly, the dorsal with large spots on the interradial 
