GENERAL RE3IARKS ON THE PIMELODIN^. 341 



tween t\}e first dorsal and the caudal, while it is very low 

 at both its extremities.* Another small group has the 

 adipose fin much shorter, and triangular : the cirri are 

 only four; and the anterior rays of the first dorsal, pec- 

 toral, and caudal fins greatly elongated. t A third as- 

 semblage have the adipose fin very small, oval, and 

 pedunculated ; and here the number of cirri varies from 

 eight to four. X it deserves some attention, that, in the 

 descriptions of the above, notice is generally taken of a 

 strong sharp spine, attached to the gill covers, — a charac- 

 ter which does not appear to be found in those Pime- 

 lodince of Tropical America which have been described 

 and figured by Spix and Agassiz. We feel convinced 

 that there must be some such difference between the 

 Pimelodince of these two hemispheres ; but this, as well 

 as many other points of structure, have been hitherto 

 passed over by our predecessors, so that we can come to no 

 determination on this point, at least in respect to such 

 species as we have not personally examined. As for 

 attempting to determine the natural sub-genera by the 

 serrature or smoothness of the dorsal and pectoral spines, 

 or even by the presence or absence of naked plates upon 

 the head, we have altogether failed in the attempt. The 

 admirable descriptions, indeed, of Dr. Hamilton, who 

 enumerates no less than thirty-three Pimelodi found in 

 the Ganges alone, is quite sufficient to convince the ich- 

 thyologist that the above characters are merely specific, 

 and therefore quite unfit to be employed for the con- 

 struction of sub-genera. One species, in fact, will some- 

 times differ only from another in having the pectoral 

 spine either smooth or serrated; a third, in the obvious 

 line of affinity, will have it barbed on one side only, or 

 on both ; and sometimes the side where the dentations 

 are situated, is different in one species to that of the next. 

 The same remark is applicable to the teeth, even on M. 

 Cuvier's confession; and the descriptions of Dr. Ha- 



* Gangetic Fishes, pi. 11. fig. 67. t lb. pl. 7. fig. 62. 



t lb. pl 31. fig. 57., pl. 23. fig. 60. 63. Kussell, No. 169. 



z 3 



