

globular, and bipartite; the unusual appearance of its swimbladder, relative to other 

 ageneiosids known at the time, may have contributed to the early confusion over the 

 correct taxonomic affinities of the genus. The traditional concept of an encased 

 swimbladder in the family was modified when Eigenmann (1912) described a new 

 genus, Tympanopleura, the diagnostic feature of which was a large, unencapsulated 

 swimbladder. The taxonomic status of Tetranematichthys and Tympanopleura is 

 addressed in separate sections, and it is only relevant to note here that both genera 

 have continually been recognized by most authors until relatively recently. 



As briefly discussed elsewhere (status of Tympanopleura), there are extensive 

 interspecific differences in swimbladder morphology, as well as changes that occur 

 during development. Britski (1972) was the first to present evidence that there is a 

 reduction in swmibladder size during ontogeny in at least some species of 

 ageneiosids; his evidence was based on a small series of v4. valenciennesi and 

 juveniles of an unidentified species. Ferraris (1988) briefly discussed encapsulation, 

 as observed in only a few specimens of three species, and he concluded thatv4. 

 nigricollis {= A. atronasus ?) retained a large swimbladder throughout life. My 

 observations of many specimens confirms Britski's , and to some extent Ferraris' 

 conclusions, although good developmental series are unavailable for most species. 

 In ageneiosids, as well as in many other catfishes, the relative size of the 

 swimbladder can be approximated in juveniles by study of the epaxial musculature, 

 below the region where the epiocdpital plates contact the lateral body wall, and 

 above the postcleithral process, if present. Generally, there is a relatively large, 

 opaque or slightly translucent area (corresponding to the "lateral cutaneous area" of 

 Alexander 1964) in juvenile specimens with a large swimbladder. This muscle-free 

 window, or "tympanum", generally becomes progressively obliterated during growth 

 in those species with encapsulated bladders. In species with a free swimbladder as 

 adults, the "tympanum" may remain large and visible at all sizes {t.g.,A. brevis and 



