FISHES OF THE FAMILY LOEICARIID.E. 203 



temporal plate, separating the second entirely from the latter ; lower surface of head 

 and ahdomen 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. Prae- 

 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; vertebrae 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 — i. e., 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 Chcetostomus anomalus, and in view 

 of the fact that in the very natural subgenus which is here established under the name 

 Pogonopoma for 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. Bhinelepis 

 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, in a 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 2§-4 times in the interorbital width. 



1. Supraoccipital entirely bordered posteriorly by a single scute; first dorsal ray about 

 as long as the bead ; some of the scutes carinate. 

 a. 25-26 scutes in a longitudinal series LP. guacari Lacep. 



