PLATE 70. 



POTAMOTRYGONIDAE. 



Fig. 1-2. POTAMOTRYGON CIRCULARIS. 



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



M. C. Z. 295 (Page 419). Fig. 3-4. Disceus thayeri. 



1, 3. 



2, 4. 



Dorsal view. 

 Ventral view. 



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

 skull forward, the copula, hhr', is segmented, the propterygial basalia of the pectoral fins, pnp, are strong 

 and in a single piece between the antorbital and the shoulder girdles, and the postbranchial stay, 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 processes; 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 ceratobranchials are more or less solidly anchylosed. Disceus has elongate narrow 

 opercular cartilages, op 1-5. 



f 



