NO. 3539 HAMMERHEAD SHARKS — GILBERT 13 



the differences in head width have had comparatively httle to do with 

 my decision. 



The phylogenetic conclusions may be summarized as follows: 



1. The nine species of Sphyrna, on the basis of morphological 

 evidence and electrophoretic analyses of serum proteins, likely evolved 

 from a single common ancestor. 



2. Sphyrna tucles, Sphyrna corona, and Sphyrna media, with 

 numerous identical morphological characters, some of which are 

 unique, are regarded as the most closely related members of the 

 genus Sphyrna. 



3. Sphyrna tihuro, with a nmnber of morphological characters 

 similar to those in S. tucles, S. corona, and S. media, appears to occupy 

 a phylogenetic position intermediate between these three species and 

 other members of the genus Sphyrna. Therefore, because of their close 

 relationship, the four species are placed in the subgenus Platysqualus. 



4. Sphyrna blochii possesses several characters that are markedly 

 different from those found in other hammerheads, such as an ex- 

 tremely expanded head, an outer narial groove, a reduced number of 

 vertebrae, and enlarged vertebrae over the posterior part of the 

 coelom. It is placed in a separate subgenus, Eusphyra, although 

 elevation of this taxon to the generic level might be justified. 



5. The interrelationships of Sphyrna zygaena, S. lewini, S. couardi, 

 and S. mokarran are more difficult to determine, although zygaena, 

 lewini, and couardi seem to show a closer relationship to each other 

 than to mokarran. The most important feature shared by the four 

 species is their large maximum size. They are referred to the sub- 

 genus Sphyrna. 



Growth changes. — -Certain modifications in shape of the head of 

 sphyi-nids occur with increase in size and age, for which there is 

 nothing comparable in other carcharhinid sharks. In young hammer- 

 heads, particularly embryos, the anterior margin of the head is more 

 or less broadly rounded, becoming gradually straighter with increase 

 in size and age until sometimes, as in Sphyrna mokarran and Sjphyma 

 tudes, this margin is nearly straight (figs. 7a and 20c). In Sphyrna 

 blochii, which has the most widely expanded head of any member of 

 the genus, the lateral extensions of the head in embryonic and early 

 postembryonic specimens are nearly parallel to the body axis (fig. 

 5c). With an increase in size of the individual, these structures 

 become situated nearly perpendicular to the body (fig. 56). It has 

 long been thought that such changes in head shape would make 

 hammerhead taxonomy a difficult problem. Such is not the case. 



Those head characters of greatest importance in distinguishing 

 species of sphyrnids, such as structure of the chondrocranium, nature 

 of the intranarial and extranarial grooves, and pattern of the mucal 



