416 THE PLAGIOSTOMIA. 



cartilage. Mouth small, transverse, with papillae at the bottom. Teeth smallJ 

 in a band or pavement. Nostrils large; anterior valves meeting on a narrow" 

 isthmus, reaching the mouth. Tail tapering to filamentary, with lateral longi- 

 tudinal folds on the base with vertical fin-folds on the distal portion, and in 

 most genera, if not all, with a serrated spine. Anterior copula of the branchihyal 

 cartilages segmented. Pelvis with a median prepelvic process. 



Mouth at a moderate distance from the anterior margin 

 disk circular 



base of tail elongate; caudal spine remote 



spiracular process absent . . Potamotrygon (page 416) 



base of tail short; no caudal spine . Elipesurus (page 424) 



Mouth far from the anterior margin 

 disk elongate elliptical 



base of tail short; caudal spine proximal 



spiracular process present . . . Disceus (page 426) 



Potamotrygon. 



Potamotrygon Garman, 1877, Proc. Bost. soc nat. hist., 19, p. 210. 

 Dynatobatis Larrazet, 1886, Bull. Soc. Geol. France, ser. 3, 14, p. 258. 



Disk subcircular. Mouth, eyes, and spiracles rather nearer to the anterior 

 margins than to the shoulder girdle. Spiracles without a process on the outer 

 border. Mouth with papillae at the bottom. Ventrals subtriangular; pelvis 

 with a prepelvic process in the middle. Lateral system of the lower surface 

 not retiform. Tail with elongate depressed base, a remotely placed serrated 

 spine, with cutaneous folds above and below, and with a low keel along each 

 side of the base. 



Species have been described from the South American Tertiary in the Rio 

 Parana Section. 



Disk nearly round 



base of tail equal more than half the length of body 

 mouth abruptly curved; teeth unequal 



interspiracular width nearly equal length of snout 



laticeps (page 417) 

 mouth waved; teeth subequal 



interspiracular width one third length of snout scohina (page 418) 



