SOUTH AMERICAN NEMATOGNATHI. 397 



eri eas adjunxi." This leaves in his genus Plecostomus: 

 Nos. 393, PL dor so dipterygio: ore dentata; cirris pluri- 

 mis, etc., = ? and 394, PL. plecostomus. Bleeker first 

 restricted the name Plecostomus as it stands at present. 



Hypostomus Lacepede and Hypostomus Guv. & Val. are 

 identical in so far as they have plecostomus for their 

 type. Hypostomus Lac. is composed of a single species 

 while Cuv. & Val. include in their Hypostomus species 

 of several genera. 



ANALYSIS OF THE SPECIES OF PLECOSTOMUS. *t 



a. Head with three distinct ridges, snout pointed, head usually elevated, 

 occipital plate ending in a pointed occipital crest. 

 6. Belly entirely covered with small granular plates. 



c. Posthumeral ridge becoming strongest on the tail which is flat be- 

 low; outer caudal rays greatly produced; occipital plate margined 

 by several small plates which become coossified with it in the 

 adult. Tip of snout naked or granular. emarginatus 344. 



cc. Posthumeral ridge strongest in front, lower surface of the tail 

 similar to its upper surface. 



d. Free margins of the lateral plates covered with long spines; eye 

 9| in the head; tip of snout granular; back with a median depres- 

 sion. Lat. 1. 32. (Steindachner.) spinosissimus 345. 

 dd. Margin of the lateral plates with short spines or none. 



e. Tip of snout naked; Lat. 1. 28-30; occipital plate margined by 

 a single large nuchal plate except in carinatus. 

 f. Spots on the caudal as distinct as those on the dorsal. 

 g. Lower surface of the head covered with scutes. 



h. Spots all small, as many as 20 on one of the anterior 

 lateral scutes; belly plain. commersonii 346. 



hh. Spots rather large, not more than 4 on one of the an- 

 terior scutes, forming 4 or 5 longitudinal series on the 

 tail; belly with similar spots. commersonii affinis 346. 

 gg. Lower surface of the head naked except a triangular spot 

 in front of the gill-openings; spots large as in affinis; oc- 

 cipital process shorter than in commersonii. 



commersonii scabriceps 3466. 



ff. Spots on the caudal less distinct than those on the dorsal; 

 all the scutes with high keels; nuchal plates bicarinate. 



*Doubtful species of this genus are Plecostnmus robini Gill, not of Cuv. & Val. 



Hypostomus commersonii Kner, not of Cuv. & Val. 



Hypostomus punclatus Kner, not of Cuv. & Val. 



tNo. 352, Plecostomus virescensCope, we have not been able to assign to a place in this key. 



