334 Me?notr Sears Foundation for Marine Research 



3a. Disc not more than 1.3 times as broad as long; tail from center of cloaca 

 to tip longer than breadth of disc; no distinct dorsal fin; transverse cur- 

 tain on roof of mouth with fringed margin ; floor of mouth with several 

 fleshy papillae. 



4a. Pelvis not bearing a slender median process directed forward; salt- 

 and brackish- water species. Dasyatidae, p. 335. 



Figure 80. Gill arch (above) and lateral view of gill fold (below) of Myliobatoidea; left, Myliobatis goodei, 

 990 mm wide, from Rio de Janeiro, Brazil (Harv. Mus. Comp. Zool., No. 994), 0.65 X; right. Rhinoptera 

 bonasus, 11 \ mm wide, from Rio de Janeiro, Brazil (Harv. Mus. Comp. Zool., No. 374), about 1.3 X. 



4b. Pelvis bearing a long slender median process, directed forward; 

 fresh-water species exclusively. Potamotrygonidae. 



Rivers of Atlantic and 

 Caribbean watersheds in 

 tropical and warm-tem- 

 perate South America. 

 3 b. Disc more than 1.5 times as broad as long; tail from center of cloaca to tip 

 considerably shorter than breadth of disc; with or without a small dorsal 

 fin near midlength of tail; transverse curtain on roof of mouth smooth- 

 edged; no papillae on floor of mouth. Gymnuridae, p. 395. 

 2 b. Margins of pectorals deeply indented or entirely interrupted j'ust posterior to 

 eyes, the anterior part of head thus sharply marked off from remainder of 

 disc; anterior subdivisions of pectorals forming a separate lobe (or lobes), 

 or fins; eyes and spiracles on sides of head. 

 5 a. Anterior subdivisions of pectorals in the form of two thin and narrow 



