224 MR, C. TATE REGAN ON THE 
Interorbital width 22 times in the length of head . . . . . . 16. A. schomburgkii Gthr. 
luterorbital width 3} times in the length ofhead . . . . . . 17. A. megacephalus Gthr. 
b. Length of mandibular ramus greater than inter- 
orbital width; «© s. » » » 0 6 « » . » . 18. A. platyecephaius Bier. 
II. Interoperculum armed with slender spines with hooked apices, which can be everted and 
completely retracted beneath the operculum; sides of head without bristles; last dorsal 
ray connected by a well-developed membrane either to the 3 or 4 scutes following it or to 
the spine of the adipose fin. (Parancistrus Blkr.) 
A. Last dorsal ray not connected to the adipose fin . . . 19. A. niveatus Casteln. 
B. Last dorsal ray connected to the adipose fin. 
Interorbital width 3 times in the length of head ; caudal peduncle 
twice as Jong as deep - ss ee « se 2 20. A. punctatissimus Stdr. 
Interorbital width 22-22% times in the length of head; caudal 
peduncle about 1} times as long asdeep . . . . . . . 21. A. aurantiacus Casteln. 
1II. Interopereculum armed with moderately stout spines, which can be everted and completely 
retracted beneath the operculum ; external to these some slender bristles; sides of the 
snout with or without short bristles; last dorsal ray connected to the seute following it 
by an inconspicuous membrane at its base. (Lasiancistrus, subgen. nov.) 
A. Bristles longer than the interopercular spines. 
Interoperculum with about 12 spines; pectoral spine extending 
beyond middle of ventral; interorbital width 1% times in the 
lenethiofihead <2) 2. eee ee a . . 22: A. heteracanthus Gthr. 
Interopereulum with 12-15 spines ; pectoral spine elongate ; inter- 
orbital width 2} times in length of head; large round light 
spots on head, body, and fins . . . .. .- . - + « . 23. A. pictus Casteln. 
Interoperculum with about 20 spines; pectoral spine extending a 
little beyond base of ventral; interorbital width 24 times in 
the length of head ; body with dark spots or markings . . . 24. A. mystacinus Kner. 
B. Bristles shorter than the interopercular spines. . . . 25. A. guacharote C. & V. 
IV. Interoperculum armed with slender spines which cannot be completely retracted beneath 
the operculum, or with permanently everted bristles ; sides of the head with bristles which 
are always longer in the male than in the female and may be wanting in the latter; last 
dorsal ray connected to the seute following it by an inconspicuous membrane at its base 
or by a more developed membrane to the 2 scutes succeeding it. Abdomen completely 
naked. (Pseudancistrus Blkr.) 
A. Interoperculum armed with spines; dorsal with I 7-8 
rays; length of mandibular ramus greater than the 
interorbital width. ..< “Queena 6 = » - sp aeeOseed. weronua Dar, 
