118 MEMOIRS OF THE CARNEGIE MUSEUM 



{histrix of Schomburgk) ; sides of tail banded, lower surface of tail marbled: a small 

 black spot near the middle of the belly. 



Paratrygon Dumeril. 

 Paratrygon Dumeril, Hist. Nat. Poiss., I, 1865, 594. 

 Disceus Garman, Proc. Bost. Soc. Nat. Hist., 1877, 208. 



Type, Raja orbicularis Bloch and Schneider. 



This genus is distinguished from Potamotrygon, among other characters, by the 

 absence of papilla? in the mouth and by the smaller number of teeth. 



2. Paratrygon orbicularis (Bloch and Schneider). 

 " Aiereba" Marcgrave, in Piso, De India?, etc., 1658, 175 (copied by Jonston, De 



Pise, pi. 38, fig. 6, and Willughby, 68, pi. 101, fig. 2). 

 Raja orbicularis Bloch and Schneider, Syst. Ichth., 1801, 361. 

 Trygon orbicularis Gunther, Catalogue, VIII, 1870, 482 (copied). 

 Trygon strongyloplera Schomburgk, Fishes Brit. Guiana. II, 1842, 183, pi. 22 



(Rio Branco). — Muller and Troschel, in Schomburgk, Reisen, III, 1848, 642 



(Rupununi; Takutu; Rio Branco). — Gunther, Catalogue, VIII, 1870, 476 



(copied) . 

 Trygon aiereba Dumeril, Hist. Nat. Poiss., I, 1865, 592 (Brazil). 



No specimens were secured. 



Disk ovate, scarcely longer than broad; eyes very small; plain or with un- 

 dulating stripes. 



Class TELEOSTOMI. 



Key to the Orders of Teleostomi found in the Fresh-waters of Guiana. 

 a. Weberian ossicles present; the anterior vertebrae coalesced. A single rayed dorsal fin, usuallyfollowed 

 by an adipose fin. 

 b. Skin naked or with bony plates; maxillary rudimentary, forming the base of a barbel.. Nematognathi. 

 bb. Scales well-developed (rarely naked); maxillary not forming the base of a barbel. 



c. Body not eel-shaped (an adipose fin usually present) Heterognathi. 



cc. Body elongate, more or less eel-shaped, the anal fin very long Glanencheli. 



aa. No Weberian ossicles; the anterior vertebra not united. 



d. Air-bladder connected with the alimentary canal by an open duct; ventral fins abdominal; no 

 spinous dorsal. 



e. A mesocoracoid; body not eel-shaped Isospondyli. 



ee. No mesocoracoid; parapophyses coossified with centra. 

 /. Body eel-shaped, no paired fins; no separate caudal. 



g. Gill-opening single, ventral; premaxillaries normally developed Symbranchii. 



(gg. Gill-openings not ventral; premaxillaries rudimentary or wanting. Apodes.) 2 



ff. Body not eel-shaped; paired fins; a well-developed caudal fin Microcyprini. 



2 Midler and Troschel (Schomburgk, Reisen, III, 1848, 639) record Gymnothorax ocellatus from 

 plantation drains, and the fresh-water eel probably enters the Guiana rivers at times. 



