362 FISHES OF WESTERN SOUTH AMERICA 
Widely disseminated within its narrower boundaries which appear to be about 
the northern fringe of Lake Titicaca and its more lacustrine tributaries, but not so 
generally distributed as O. agassizii or miilleri. One of the most extreme and 
distinctive species. The following paragraphs are descriptive mainly of my largest 
specimen, 172 mm. in length, part of 16095, collected in Lake Umayo, at the foot 
of the mountain Atuncolla with its ancient ruins. (Fig. 42). 
D. 13; A. 18; P. 20; C. 28; vertebral series 12. 
Head in length 3.3 and equal to greatest depth; its width slightly less, 3.5 in 
length; crown strongly convex, depressed at occiput; snout slightly greater than 
diameter of eye, which, although large, is contained in the large head 5.8 times; 
more than 2.0 in the broad interorbital; the orbit tangent to the crown, with a 
shghtly prominent overhang; snub-nosed; head extremely angular; mandible ver- 
tical, gape horseshoe-shaped, front view, the rictus reaching little below the orbit; 
width of gape 1.0-1.4 in the interorbital. 
Dorsal profile strongly arched from occiput to dorsal fin; chin blunt, mouth 
superior, thin-lipped, edentulous. Postorbital portion of head 1.7 times greater 
than snout plus eye, the broad opercle terminating in an almost right angle. 
Scalation complete, no naked area on back; a row of bucklers elevated into a 
ridge above the opercle; well-developed granulations on the bucklers to the middle 
of the pectoral, more or less worn off at shoulder; remaining scales outlined with a 
brown, faintly granulate zone, which gives to the trunk a reticulate appearance; 
prepectoral with three rows of scales extending well below the base of the fin; 
sides also well-sealed, except for a naked patch extending from the belly up to the 
pectoral basis; dorsal, anal, and pectoral fin bases elevated and naked. 
A large, fleshy, protuberant area encloses the vent, the openings prominent. 
The fins are of the flexible, membranous, harsh type, with a brown membrane 
between rays. Dorsal and anal short, erect, rounded; caudal broader than long, 
slightly rounded; pectoral broad, rounded. 
Younger specimens down to 30 mm. in length show the predorsal hump and 
enlarged scales, without granulations. Width of the head only moderate, 1.3-1.5 
in its length, compared to 1.0 in the largest specimens, in which width, depth, and 
length are about equal. The smaller ones have a maximum body depth of around 
3.3 as compared with 2.8 in the larger. 
Profile of head nearly straight above, the frontal eminence not appearing in 
specimens of less than 150 mm. Eye in head 4.0-4.8; in snout 1.0; in interorbital 
1.8; those of intermediate size flat on the crown. 
Caudal peduncle greatly elevated and compressed, equal to the postorbital 
part of the head; opercle more acutely angular in the young; mouth smaller, two 
rows of teeth still present and well developed up to the 100-mm. specimens, more 
or less obsolescent in those of 1830 mm. or larger. 
ID. A3=165 AS14215. Pe 19229 C>-27-28) 
General ground color at all ages a flat yellow, locally silvered, the scales be- 
coming outlined in brown (in alcohol), and the general color darker above in- 
creasingly with age. 
