12 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 119 



brae situated over the posterior part of the coelom are all unique to 

 this species, and it is consequently placed in a separate subgenus, 

 Eusphyra. S. blochii is so different from the other hammerheads that 

 recognition of Eusphyra as a genus would not be unwarranted. 



Contrary to the relative ease with which the relationships of the 

 preceding species are discerned, those of Sphyrna zygaena, S. lewini, 

 S. couardi, and S. mokarran are more difficult. The main featiu-e 

 indicating a fairly close interrelationship among these species is 

 similarity in maximum size. All reach a total length of at least 12 

 feet, more than twice that attained by any other members of the genus. 

 S. zygaena and S. lewini (and probably S. couardi) have certain 

 featui-es in common, such as a low second dorsal fin with a long lobe; 

 a short anal fin base, the length of which is never more than slightly 

 greater than the pectoral base; a long slender anterior part of the 

 preorbital process of the chondrocranium; a deeply falcate anal fin; 

 and a deep, widely V-shaped upper precaudal pit, with a definite ridge 

 on the anterior margin. This suggests that they probably are more 

 closely related to each other than to S. mokarran (fig. 4, table 1, 

 and figs. 6-10). 



The following examples, however, illustrate the difficulties one 

 encounters in attempting to determine the relationships of *S'. zygaena, 

 S. lewini (including S. couardi) and S'. mokarran: The overall shape of 

 the rostral node, anterior-median pore patch, and anterior part of the 

 preorbital process in the chondrocraniiun of Sphyrna zygaena are most 

 similar to those found in S. blochii; S. zygaena and S. mokarran are 

 the only hammerheads lacking a wing on the olfactory cartilage; S. 

 lewini and S. mokarran are the only members of the genus that ordi- 

 narily have a rostral fenestra; S. mokarran and S. blochii have, as do S. 

 tudes, S. media, S. corona, and S'. tiburo, a rather tall second dorsal fin; 

 S. mokarran has, as do S. tudes, S. media, S. corona, and S. tiburo, a 

 blunt, clublike knob at the end of the anterior part of the preorbital 

 process of the chondrocranium; and S. mokarran is miique, among the 

 hammerheads, in having a deeply falcate pelvic fin. 



Despite the difficulties encountered in interpreting the relationships 

 of the above foiu- species, they appear to be related more closely to one 

 another than to any other group of species of Sphyrna and thus are 

 included in the subgenus Sphyrna. 



In the preceding discussion, nothing was said regarding head width, 

 although this is almost the only basis for previous separations of the 

 genus Sphyrna into separate genera and subgenera. For example, 

 Tortonese (1950a, pp. 3-6) placed Sphyrna tiburo in the subgenus 

 Platysqualus and placed Sphyrna blochii in a separate genus, Eusphyra. 

 Although I have followed both of these allocations (in modified form), 



