A.piperatm), although this is not the case iaA.pardalis, and in adults of A. atronasus 

 and Tetranematichthys the "tympanum" is relatively small and obscured by heavy 

 pigmentation of the overlying skin. 



The surface of an encapsulated swimbladder, and in some cases the 

 longitudinal septum, is easily observed in radiographs, thus providing a direct 

 method of measuring its size without necessitating dissection of the specimen. In 

 species with an encapsulated swimbladder, the swimbladder is proportionally large 

 at relatively small body sizes (< 50 mm SL), but there is strong negative allometry 

 with increasing SL (Fig. 19). Encapsulation proceeds by ventrolateral laminar 

 ossifications of the complex centra, which progressively encircle the swimbladder on 

 the sides and anteriorly, as discussed in greater detail below. In A. ucayalensis, the 

 swimbladder is fully encapsulated at a relatively small size, and the negative 

 allometry is due entirely to somatic growth. In other species {t.%.,A. vittatus), 

 allometry is due to a combination of an increase in SL and an overall slower rate of 

 encapsulation. The posterior caecae may or may not become thinly ossified, 

 depending on the species and size of the specimen. Ferraris (1988) reported that a 

 single 85-mm SL specimen of ^4. marmoratus had a relatively large, unossified 

 swimbladder, and that encapsulation in this species began to occur around 170 mm 

 SL. However, I observed a much smaller specimen oiA. marmoratus (FMNH 

 96585; 83 mm SL) with an almost fully encapsulated swimbladder (I have not 

 verified the identification of Ferraris' specimens). 



;, The swimbladder itself, or the encapsulated capsule, varies widely in 

 structure among species, and is of considerable taxonomic use (Figs. 20-21). 

 Primitively, the siluroid swimbladder consists of a relatively large, flexible sac 

 corresponding to the anterior chamber of other ostariophysans, and with a 

 characteristic arrangement of internal transverse and longitudinal septae and 

 surrounding epithelial layers (Alexander 1964, 1965). In some ageneiosids, and a 



