HAMMERHEAD SHARKS GILBERT 



55 



Pacific Ocean: Costa Rica: USNM 196140 (6), Gulf of Nicoya. 

 Mexico: UCLA 58-47 (2, including one bead), Gulf of California, 

 south of Bahia Topolobampo, off Isla San Ignacio and Isla Macapule, 

 Sinaloa; FMNH 63093 (2), above San Benito, Chiapas; SIO 60-88 

 (3), Gulf of California, 21°55' N., 105°37' W.; SU 11583 (holotype of 

 Sphyrna media — partial skin), Mazatlan, Sinaloa; USNM 28160 (1 

 paratype of Sjihynia media), Mazatlan, Sinaloa. Panama: USNM 

 190593 (4), UCLA 58-304 (4, includmg three heads), (all originally 

 UCLA 58-304), Panama Bay, offshore between Punta de Hicacal 

 and Rio Pasiga; USNM 50376 (1), no further data. 



Initially Sphyrna media and Sphyrna corona were regarded by me as 

 synonymous. This conclusion was based mainly on the fact that dif- 

 ference in anterior contour of the head, which was the primary basis 

 for Springer's origmal separation of the two forms, does not remain 

 constant, the head shape grading from the more evenly rounded con- 

 dition of S. media to the more lobate situation found in S. corona. In 

 addition, the chondrocrania of the two forms are very similar, as are 



Map 4. — Distribution of subspecies. 



9 Sphyrna tiburo tiburo 



A Sphyrna tiburo vespertina 



(Solid = specimens examined; hollow: 



confirmed literature references.) 



the various external morphological features. Closer examination of the 

 chondrocrania, however, revealed several relatively smaD, but none- 

 theless constant, differences. With a firm basis for separation thus 

 available, it was found that other morphological differences, which 

 were at first thought to be due to individual variation, also could be 

 used to separate the two species. The various distinguishing features 

 are discussed in the diagnoses of the respective species. 



Diagnosis. — Characters mentioned in the diagnosis of the subgenus 

 Platysqualus are not repeated here. 



Sphyrna media differs from the closely related S. tudes in the fol- 

 lowing characters: Anterior margin of head more rounded (partic- 

 ularly in large specimens), with no pronounced median notch; inner 

 narial groove absent; anterior-median pore patch on underside of head 

 with the posterior margin more broadly rounded than in ^S. tudes and 

 with a pointed median-posterior extension (fig. 22/) ; narial depression 

 very shallow; first dorsal fin of larger specimens (600 mm. or more) 

 less erect and less broadly triangular, a line drawn perpendicularly 

 from apex of fin not intersecting base of fin. 



