Fishes of the Western North Atlantic 



429 



at base, with low lateral keel on each side; its length from center of cloaca about 1.2 

 times as great as distance from center of cloaca to tip of snout. 



A single tail spine, serrate along distal part; its extreme length nearly as great as 

 length of snout in front of eyes; its origin a little less than halfway (about 42 %) back 

 from level of axils of pectorals toward tip of tail. 



Skin, apart from tail spine, smooth throughout. 



Figure ioi. Urotrygon microphthalmum,]uvem\e 

 male, 220 mm broad, from off the Amazon River; 

 after Delsman. 



Snout in front of eyes about 2.2 times as long as distance between eyes. Eye 

 noticeably minute, much smaller than spiracle. Spiracles close behind eyes, their length 

 about 1/3 as great as distance between eyes. Nasal curtain fringed. Mouth slightly 

 arched. 



Teeth small, pointed. 



Caudal fin much narrower relatively than in Urolophus jamaicensis (cf. Fig. loi 

 with 99), its upper and lower origins indefinite; lower about even with upper, posterior 

 to tip of tail spine by a distance a little more than half as great as distance between eyes. 

 Pelvics reaching rearward beyond posterior limits of pectorals for a distance about as 

 great as distance between eyes, their anterior margins nearly straight, posterior margins 

 weakly convex; outer and posterior corners angular, a little less than 90°. Claspers of 

 mature male not seen. 



Color. Upper surface gray. Lower surface whitish, the disc edged with gray. 



