222 MR. C. TATE REGAN ON THE 
Plecostomus (part.) Giinth. Cat. Fish. v. p. 230 (1864). 
Liposarcus Giinth. t. ec. p. 238. 
Chetostomus (part.) Giinth. t. ec. p. 240. 
Delturus Kigenm. & Kigenm, Occ. Pap. Cal. Ac. i. 1890, p. 437. 
This genus differs from Plecostomus essentially in the structure of the operculum and 
interoperculum, which are independently movable; the latter opens outwards and 
forwards, and usually bears some spines, which are thus everted, and can be retracted 
beneath the operculum, which opens outwards and upwards. In some species these 
spines have become lost, in others they are replaced by long bristles which cannot be 
retracted beneath the operculum. Dorsal with I 7-15 rays. An adipose fin is con- 
stantly present. Vertebre 54+10+418 (in A. multiradiatus). 
tivers of S. America. 
It has not seemed advisable to place the closely allied A. annectens and A. undeci- 
malis in different genera because of a slight difference in the number of dorsal rays, and 
this applies to A. medians and A. gigas and also to A. barbatus and A. angulicauda. 
In most of the species the last dorsal ray is connected by a membrane at its base to 
the scute following it; in A. niveatus this membrane is enlarged, and is attached 
to three scutes instead of to one, this species leading to those which have been 
regarded as forming a distinct genus, Parancistrus, in which the last dorsal ray is 
connected by a membrane to the spine of the adipose fin. 
A, setosus is intermediate between the species in which the interoperculum is armed 
with slender spines which can be retracted beneath the operculum, and A. barbatus 
and its allies, in which the interoperculum is armed with long bristles which are 
permanently everted, the sides of the snout being furnished with similar bristles. The 
interopercular spines in A. setosus are long, but similar in structure to those of 
A. oligospilus, &c., and they can be folded back and their basal portion concealed 
beneath the operculum, whilst their free ends project ; the sides of the snout are bristly 
in the male, but not distinctly so in the female. 
Key to the Species. 
I. Interoperculum usually 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 to the scute following it by an inconspicuous membrane at its base. 
(Ancistrus.) 
A. Occipital with more or less distinct median ridge ; snout narrowed anteriorly ; some of 
the scutes carinate; lower surface of head and abdomen, in the adult, completely 
covered with small granular scales. (Species with the general form of Plecostomus 
guacari.) 
1. Dorsal-with LY xrays’ 2 sss & 4s. suena 0 arinectens ome 
