observed to rest on debris or on the bottom at negative buoyancy (L. Nico and L. 

 Finley, personal communication), except when foraging for prey. Under natural 

 conditions, however, they have been observed to actively swim in the upper layers of 

 the water column and forage at the surface (L. Nico, personal communication). 



The compressed body of ageneiosids is superficially similar to that of many of 

 the taxa discussed above, but the combination of a compressed trunk and the 

 extremely flattened head with ventrolaterally displaced eyes is unique among 

 catfishes. In some auchenipterids and in Hypophthalmus, the body is compressed 

 and the eyes are ventrolaterally positioned, but none of the species in these groups 

 have the head depressed to the extreme degree XhoXAgeneiosus does. I beheve that 

 the uniquely combined depression of the head and compression of the body is a 

 derived condition characteristic only of the family Ageneiosidae. Lundberg (1982) 

 suggested that a greater depth of the shape of the head and anterior part of the body 

 are correlated, at least in ictalurids, and probably is a shared primitive condition 

 within catfishes in general. Tetranematichthys has a relatively deep body and more 

 dome-shaped head, superficially resembling some auchenipterids (e.g., 

 Trachelyopterus spp.) in general body shape. However, aspects of the neurocranium 

 and a number of other internal features of Tetranematichthys are clearly more like 

 Ageneiosus than in the auchenipterid species that it resembles. Among 

 auchenipterids and ageneiosids, there is a large clade of species that is defined by a 

 relatively long anal fin and a laminar epioccipital, assigned to the subfamily 

 Auchenipterinae by Ferraris (1988). Within this group, there appears to have been 

 a trend toward greater elongation and compression of the body, and some degree of 

 flattening of the hedid; Ageneiosus and Tetranematichthys appear to be further 

 derived from the condition present in all other taxa of this lineage. The differences 

 in head shape are correlated with osteological differences of the neurocranium, 

 discussed in greater detail below. ' 



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