eigenmann: the pygidiid.e, a FAMILY OF SOUTH AMERICAN CATFISHES. 277 



another shorter barbel just beneath the maxillary barbel; this lower labial barbel is 

 sometimes very minute and has been overlooked in describing some species of 

 Henonomus and Pseudostegophilus, and in some species of other genera. 



Mental barbels, characteristic of many Nematognaths, are lacking, except in 

 Nemafogenys. Thorn-like spines (irmly attached to the opercle and the interopercle 

 in all but Nematogenys. The opercles and interopercles to which the spines are 

 attached are erectile, and by first erecting those on one side and then those of the 

 other, the fishes are able to "elbow" their way forward in narrow openings, under 

 rocks and up waterfalls. In some cases the spines are directed backward, but in 

 Vandellia the opercular spines point obliquely upward and backward, the interoper- 

 cular spines downward and backward. 



All of the species secrete a copious mass of mucus, and the larger, ones are as 



J H0M0DI£TUS 



ZlCANTHOPOAIA 



8 PSEUDOSTE&OPH 

 1 HENONEAAUS 



L. PAREIODON 

 SEREMOPHlLUS-stt 



4PYCIDIUM 



3 HATCHERIA 

 iSCLERONEMA 



hmacanthus 

 13 Vandellia 

 Urinophi lus 



i5paravandellia 



fcBRANCHIOICA 



I7TRIDENS 

 I8/V\!UR0GLANIS 



Nematogenys . 



TWrruittt'v* +y i 



Fig. 1. Phylogenetie tree showing the relationship of the Pygidiidce. The letters correspond to 

 the letters in the key to the subfamilies and genera. The Nematogenyince arc undoubtedly the most 

 primitive of the family. The Pygidiince have the family characteristics fully developed. Beyond these 

 we have the more highly specialized subfamilies, culminating in the parasitic StegophilintB and the uri- 

 ne iphilous Y andelliiiwB. 



