224 MR. C. TATE BEGAN OX THE 



Interorbital width 2f times in the length of head 16. A. schomburgldi Gthr. 



Interorbital width 3| r times in the length of head 17. A. megacephalus Gthr. 



b. Length of mandibular ramus greater than inter- 

 orbital width 18. A. platycephalus B\gr. 



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 long as deep 20. A. punctatissimus Stdr. 



Interorbital width 2f-2f times in the length of head; caudal 



peduncle about l.V times as long as deep 21. A. aurantiacus Casteln. 



III. Interoperculum 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 scute 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 If times in the 



length of head 22. A. heteracanthus Gthr. 



Interoperculum with 12-15 spines ; pectoral spine elongate ; inter- 

 orbital width 2} s 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 2-1 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 w r ith 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 scute 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 26. A. setosus Blgr. 



