56 Memoir Sears Foundation for Marine Research 



Rhinobatos horkelii (Miiller and Henle) 1841 



Brazilian Guitarfish 



Figures 12D, 13A, B 



Study Material. Four specimens, male and female, 525 to 840 mm long, from Rio 

 de Janeiro, Brazil, in Harvard Museum of Comparative Zoology. 



Distinctive Characters. R. horkelii closely resembles both R. lentiginosus and R. per- 

 cellens in general form. However, its nostrils are longer relatively than in either of 

 these; its crown is flat transversely or even slightly convex (definitely, if only slightly, 

 concave in R. lentiginosus and in R. percellens); its tubercles in the median row are 

 larger, more thorn-like, and more numerous; the origin of its first dorsal is posterior 

 to the tips of Its pelvics by a distance about equal to the base of the first dorsal (by a 

 distance about equal to the extreme length of the first dorsal from origin to rear corner 

 in the others); and its teeth are fewer in number than in the other two species. Its plain 

 coloration is a field mark which sets it off from most specimens of R. lentiginosus and 

 R. fercellens. Its narrower rostral projection distinguishes it further from R. lentiginosus. 

 And the anterior nasal flap differs in detail in these three species (Fig. 12). 



Description. Proportional dimensions in per cent of total length. Male, 705 mm, 

 and female, 790 mm, from Rio de Janeiro (Harv. Mus. Comp. Zool., No. 544). 



Disc: extreme breadth 33.0, 34.7; length 38.6, 40.3. 



Snout length: in front of orbits 15.2, 16.8; in front of mouth 17.7, 19.2. 



Orbits: horizontal diameter 2.8, 2.7; distance between 3.9, 4.2. 



Spiracles: length 2.1, 1.9; distance between 5.1, ^.z^. 



Mouth: breadth 5.4, 5.8. 



Nostrils: length 3.5, 3.7; distance between inner ends 2.4, 2.5. 



Gill openings : \e.n^t\i?., ist 1.8, 1.6; 3rd 1.8, 1.8; 5th i.i, 1.5; distance between 

 inner ends, ist 12.2, 13.0; 5th 8.9, 9.6. 



First dorsal fin: vertical height 6.4, 7.2; length of base 5.1, 4.8. 



Second dorsal fin: vertical height 6.0, 6.8; length of base 5.7, 5.6. 



Caudal fin: upper anterior margin 15.0, 15.6. 



Pelvics: origin to tip 14.5, 16.0. 



Distance: from tip of snout to center of cloaca 40.8, 42.7; from center of cloaca 

 to tip of tail 59.2, 57.3; from tip of snout to ist dorsal 56.6, 58.8. 



Interspace between: ist and 2nd dorsals 10.5, 10.2; 2nd dorsal and caudal 

 6.3, ^.6. 



Disc about ^e ^s broad as long to posterior limits of pectorals; wedge-shaped 

 anteriorly but with well rounded tip, its angle in front of orbits about 50—60°; antero- 

 lateral contours slightly concave anterior to level of eyes, posterior margins evenly and 

 moderately convex, posterior corners well rounded. Tail from center of cloaca 1.3-1.4 

 times as long as distance from snout to cloaca, nearly flat below, moderately rounded 

 above, tapering evenly rearward, its breadth opposite axils of pelvics about twice as 



