EIGENMANN: the PYGIDIID^, 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 

 Hcnonomus and Pseudostcgophiliis, and in some species of other genera. 



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

 Nematogenys. Thorn-like spines firmly attached to the opercle and the interopercle 

 in all but Nematogenys. The opercles and interoperclcs 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 



8 PSEUDOSTEGOPHILUS 

 1 HENONEMUS - '^ 



.1 HOMOO/^TUS g^^^^p^, M/1C.NTH0POA.A 



I OCHMACANTHVS 



bPARE10D0N-~^l 



5ERE/W0PHILUS-^t 



4PYC1DIUM 



li Vandellia 



/4()f?IN0PHI LUS 



15 Paravanoellia 



Vn >-n ">^i 



IIoBRANCHIOICA 



iiTridzns 



8/VUUROC.LANlS 



3 HATCHERIA 

 iSCLERONEMA 



I Nematogenys , 



Fig. 1. Phylogeiietic tree showing the relationsliip of the Pijgidiidw. The letters correspond to 

 the letters in the key to the subfamilies and genera. The Ncmalogcnijince are undoubtedly the most 

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

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

 nophilous VandelliincB. 



