""^isol''] proceedings of the national museum. 7 



stock. The maxillary, iu this family, bears teeth ami forms part of the 

 mouth border. Only two short barbels are present. In all other families 

 of this order the maxillary is vestigiary, its sole function being to serve 

 as a basis for the primary barbel. Through the Tachisurince the Diplo- 

 mystidfc are very closely relatt d to the Siluridce. 



Through Ageneiosus the HijpophthalmiddJ are closely related to the 

 Siho'idw. 



The Aspredlnidce are highly specialized and are evidently an early 

 offspring from the common stock. 



The Pygidiidw are the mountain forms of the Siluridxe, but have under- 

 gone many important modifica ions. 



The Argiid(e are the mountain forms of the Loricariida\ 



The Aspredinidtc ara the most specialized of the Nematognathi. The 

 mouth and the air-bladder are greatly modified, while the body is covered 

 with small bony plates. 



The Callickth}/id(e are in some sense intermediate between the Silu- 

 ridce and the LoHcariidce. They have a normal mouth and the body 

 covered with two series of bony plates. 



The ElecfropJioridce and ISternopygidm constitue the order Gymnonoti. 



The Gymnotidw ditler from the iSfernopygidce in being naked and in 

 possessing an electric organ. The members of both families are long, 

 eel-shaped fishes without a true dorsal fin, without ventral fins, and 

 having a very long anal fin. 



None of the Percomorphi are peculiarly South American, the only re- 

 maining family being the Polyceyitridw, whose position in the system is 

 not definitely determined. 



Of the families having a wider distribution, but reaching, in South 

 America, a peculiar development, must be mentioned the marine forms, 

 which, in other regions, do not ascend much beyond brackish water, 

 but which here are found even at a great distance from the sea. Chief 

 of these are the Dasybatidw, Belonidw, Mugilidce, ScicvnidcB, Batrachidci', 

 Pleuronectidte, Tetraodon t ida\ 



Of especial interest is Lepidosiren paradoxa, which represents an an- 

 cient order of fishes. 



The Siluridce here reach their greatest perfection, forty-eight genera 

 of one hundred and ninety-nine species being found iu fresh waters, 

 while several species inhabit the surrounding seas. They are gener- 

 ally inhabitants of the low lands. The peculiarities of the Pimelodince 

 are the remote nares, which are not [)rovided with a barbel, and the 

 great development of tjie maxillary barbels. 



The Callophysime are Pimelodhue with incisor- like teeth. 



The Doradince are provided with a lateral series of bony plates. 



The Ageneioshuu have a peculiarly modified air-bladder. 



The Aiichenipterince are very closely related to the Ageneiosime, but 

 possess a normal air bladder. 



