SOUTH AMERICAN NEMATOGNATHI. 89 



Tachisurus jissus seems to differ from this species in 

 the proportions of the head. As the figure of spixii 

 in this respect agrees with the description of A. arenatus 

 C. & V., it is more probable that Agassiz figured this 

 species and not fissus , as Dr. Giirither supposed. More- 

 over, Jissus has not yet been found in Brazil, and the 

 P. spixii Agassiz is said to have come from equatorial 

 Brazil. The name Pimelodus albidus is preoccupied, 

 and the name spixii should supersede arenatus. We 

 are unable to distinguish Arius laticeps Giinther from 

 A. spixii Ag. and A. arenatus Cuv. & Val. 



Body compressed, especially toward the caudal fin; the . 

 depth greater than the width. Head narrowed forward, 

 its greatest width 1J in its length, its greatest depth 1J; 

 width at the mouth 2-J- in the length of the head. Top 

 of the head granular in the young, the granules becom- 

 ing more or less united in the adult, forming fine reticu- 

 lating ridges, especially on the occipital process; occi- 

 pital process longer than broad, with a blunt median 

 ridge, the margins concave. Fontanel narrow, without 

 interruptions, continued as a deep tapering groove to 

 near the base of the occipital process; interorbital area 

 with four ridges; opercles and humeral process rough- 

 ened, covered with skin; sides of the head and snout 

 with reticulating mucous canals. 



Eye 1J-2 in the snout, 5-6J in the head, 2|-3 in the 

 interocular. 



Maxillary barbels varying in extent, from about the 

 middle of the pectoral to the base of the ventrals; post- 

 mental barbels extending to base of pectoral or to near 

 its tip; mentals to edge of gill membrane or to beyond 

 base of pectoral. 



Upper jaw projecting; lips more or less papillose; 

 teeth, on the intermaxillary and the outer ones 



