PLATE 70. 



POTAMOTRYGONIDAE. 



Fig. 1-2. PoT.iMOTRYGON ciRcuLARis. M. C. Z. 295 (Page 419). Fig. 3—1. Disceus thaykki. 

 M.C. Z. 606 (Page 426). 



1, 3. Dorsal view. 



2, 4. Ventral view. 



The River Trygons are olose allies of the Dasybatidae. They have no rostral cartilage from the 

 skull forward, the copula, hbr', is sogmented, the jjropterygial basalia of the jjeetoral fins, pap, are .strong 

 and in a single piece between the antorbital and the shoulder girdles, and the postliranchial sta.y, pbs, 

 is reduced. Disceus, fig. 3, has a small hardly distinct antorbital cartilage, ro, a broad postspiracular 

 psp, a wide bar in the shoulder girdle and short orbital proce.s.ses; Potamotrygon, fig. 1, has a larger 

 antorbital, a narrow and elongate postspiracular, a narrower bar in the girdle and longer orbital processes. 

 In both genera the ceratobranchi.als are more or less solidly anchylcsed. Disceus has elongate narrow 

 o])ercular cartilages, op 1-5. 



i 



