FISHES OF THE FAMILY LORICARIID®. 203 
temporal plate, separating the second entirely from the latter; lower surface of head 
and abdomen naked, or with small granular scales ; snout covered with granular plates 
to its margin. Operculum and interoperculum with or without marginal bristles, the 
lower edge of the former contiguous and firmly connected by membrane to the upper 
edge of the latter, so that they are incapable of independent movement.  Pre- 
maxillaries nearly equal in length to the dentaries, and opposed to them; teeth 
numerous, slender, curved, bifid, forming a nearly straight series in each jaw. Dorsal 
with I 7 rays, originating in advance of the ventrals, separated from the supraoccipital 
by 3 (rarely 2 or 4) scutes. Anal with I 3-5 rays, pectoral with I 6, ventral with I 5. 
Caudal emarginate or truncate. Adipose fin, if present, represented by a movable 
spine and a membrane attaching it posteriorly to the caudal peduncle. Ribs present, 
slender ; vertebre 5+9+16 (in P. verres). 
Rivers of S. America. 
It has seemed most convenient to retain this genus, although the difference from 
Ancistrus is so slight that some authors have proposed to unite them, a proposal which 
is greatly strengthened by the discovery of the species described below as Ancistrus 
annectens, which can only be specifically distinguished from Plecostomus guacari 
by 3 or 4 characters, one of these, however, being that on which the generic 
distinction is based—. é., the structure of the operculum and interoperculum. ‘The 
presence or absence of an adipose fin can scarcely be regarded as of generic importance 
in the light of the great variation which exists in Chetostomus anomalus, and in view 
of the fact that in the very natural subgenus which is here established under the name 
Pogonopoma tor P. wertheimeri Stdr., P. pellegrini Rgn., and P. genibarbis C. & V., an 
adipose fin is present in the first-named species, absent in the others. LRhinelepis 
Spix is therefore placed as a subgenus under Plecostomus. It is curious to note that 
in the fairly numerous examples of guacari, P. commersonii, and P. punctatus 
on the one hand, and of P. verres and P. emarginatus on the other, which have been 
examined by the author, the supraoccipital is constantly entirely bordered posteriorly 
by a single scute in the former, by a median scute and by one or more on each side in 
the latter, this feature seeming of considerable importance in showing the natural 
relations of the species. However, ina series of twelve examples of P. wuchereri, eight 
have the supraoccipital bordered by one scute, four by three, the character here proving 
not to be of specific value. 
Key to the Species. 
I. Adipose fin present, well-developed; operculum and interoperculum not margined with 
bristles. (PLEcostomus.) 
A. Length of mandibular ramus 22-4 times in the interorbital width. 
1. Supraoccipital entirely bordered posteriorly by a single scute ; first dorsal ray about 
as long as the head; some of the scutes carinate. 
a. 25-26 scutes in a longitudinal series . . . . «. . - » + + J. P, guacari Lacép. 
