no 



Alexander (1965) equated the first two nuchal plates of catfishes with a fused 

 supraneural and dermal ossifications of the first proximal radial, and he considered 

 the second nuchal plate(s) to be formed from the second proximal radial. At 

 present, there are few data available to account for the osteological derivation of 

 the nuchal plates, but most authors have considered them to be dermal ossifications 

 associated with the anterior pterygiophores. Thus, although homology of the plates 

 among taxa has not been thoroughly established, many investigators have treated 

 them as though they are homologous. Based on the available literature, and in the 

 absence of comparative embryological material, one is left with little choice but to 

 treat the nuchal plates of doradoids as homologous, and to assume that they are 

 derived from dermal ossifications of the first three dorsal-fin pterygiophores. 



The detailed discussion of dorsal fin-ray supports and the nuchal shield 

 presented above is relevant to ageneiosids in the following important respect. Most 

 doradoids have two large, unpaired nuchal plates, and a pair of smaller posterior 

 plates. In ageneiosids (and Centromochlm among auchenipterids; Ferraris 1988), 

 however, the plate corresponding in position to the anterior one of other doradoids 

 is absent. The absence of the first nuchal plate was considered by Britski (1972), 

 Royero (1987), and Ferraris (1988) to be one of the characters separating 

 ageneiosids from all remaining doradoids. Both Royero (1987) and Ferraris (1988) 

 felt that the element that normally forms the anteriormost nuchal shield of doradids 

 and auchenipterids was fused to the posteroventral margin of the supraoccipital, but 

 neither author gave ontogenetic evidence to support this idea. 



Thus, the ageneiosid nuchal shield consists of two plates, presumably derived 

 from the second and third dorsal-fin pterygiophores. The anterior nuchal plate is 

 very large and roughly triangular, being sutured along its entire anterodorsal margin 

 to the supraoccipital (Figs. 3-6), and posteroventrally sutured with the epioccipital. 

 The posterior nuchal plates consist of two small laminar bones that are suturally 



