stubs. One juvenile of ^. marmoratus (AMNH 56068; 44.8 mm SL) has a single pair 

 of relatively long mental barbels. A series of threes, brevis (MZUSP 27818; 51.2- 

 69.0 mm SL) all have mental barbels, but they are progressively shorter with 

 increasing body size. Likewise, eight specimens (56-105 mm SL) of ^. atronasus 

 from several lots all have a single pair of mental barbels. There are no chin barbels 

 evident in a nuptial male of the diminutive species ^./jiperoft/j (ANSP 137688; 40 

 mm SL), and one can only speculate that, if chin barbels develop in this species, they 

 must appear at a very early stage. At the other extreme, some large adults oiA. 

 brevifilis have very small epidermal indentations or pores that correspond in position 

 to the mental barbels present in other species, and occasionally there are traces of 

 similar pores corresponding to the mental pair, despite the absence of any distinct 

 chin barbels in one 77-mm juvenile. In all of the specimens examined, the chin 

 barbels appear to be derived from the superficial integument, and there is no 

 evidence from counterstained specimens that there are cartilaginous supporting 

 structures, a& in Hypophthalmus and several genera of pimelodids (Howes 1983), 

 Auchenipterus (Ferraris 1988), and a number of other catfishes. More complete 

 series of juveniles of all species are needed in order to determine the developmental 

 sequence and taxonomic implications of chin barbels in Ageneiosus. 



The ontogenetic loss of chin barbels in Ageneiosus is herein considered to 

 represent a derived state relative to most other catfishes. This condition is 

 analogous to the situation in some species of the Asian family Pangasiidae, in which 

 the mental and/or nasal barbels become progressively reduced and eventually 

 disappear during growth (Karamchandani and Motwani 1956, Fumihito 1989). 

 These observations imderscore the suggestion by Fuiman (1984) that the timing of 

 barbel development is important in siluroid systematics, but it must be emphasized 

 that at present there is httle evidence to corroborate a hypothesis that pairs of chin 

 barbels are uniformly homologous among families of catfishes. Arratia (1987) 



