Garman.] 208 [May 2, 



forty-two In the lower. Anterior lateral processes of the pelvis slen- 

 der, equal in length to their distance from the middle of the shaft. 



Total length 14.6 ; from snout to extremity of ventral 9 ; from 

 snout to tail 7.7; from snout to vent 6.6 ; from snout to mouth 1.7; 

 and width of pectorals 9 inches. 



Specimen from the Mejillones. 



TRYGONID^. 



Pectoral fins continuous, united in front of the head. Skull not 

 produced into a rostral cartilage. Tail slender, tapering. A ser- 

 rated caudal spine present in most genera (absent in Anacanthus). 



Dorsal fin absent or rudimentary. 



With a subabdominal cartilage from the pelvis, inhabiting fresh or 

 brackish waters Potamotrygones. 



Without a prepelvic cartilage, marine, few species venturing into 

 fresh waters Thalassotrygones. 



POTAMOTRYGONES. 



Mouth without papillae; teeth in less than twenty-five series in 

 each band; caudal spine near the pectorals Disceus. 



Mouth with papillae; teeth in more than twenty-five rows; caudal 

 spine distant from the pectorals Potamotrygon. 



Disceus gen. nov. 



Disk oval. Head not prominent. Spiracles with a process on the 

 posterior margin. Mouth near the centre of the disk. Tail slender. 

 Caudal spine small, near the body. Peripheral vascular system anas- 

 tomosing into a network on the anterior half of the lower side. Oral 

 papillae undeveloped. With scales and tubercles. Name from 

 &<rx£t3<r, a meteor having the shape of a quoit. 



Disceus strongylopterus. 



Try yon aiereba Mull. & Henle, Plagiostomen, p. 196 (not 160). 



Trygon strongylopterus Schomburgk, Fish. Brit. Guiana, n, p. 183, 

 pi. 22. 



Trygon strongylopterus Mull. u. Trosch., Schomb. Reisen, in, p. 642. 



Trygon (Himantura) strongylopterus Dumeril, Elasmobranchs, p. 

 592. 



Trygon (Paratrygon) aiereba Dum., 1. c, p. 592. 



Trygon strongylopterus Giinth. Cat., Vin, p. 476. 



Trygon orbicularis Giinth., 1. c, p. 482. 



