NO. 3539 HAMMERHEAD SHARKS GILBERT 47 



In addition, the subgenus is characterized by: A fairly high second 

 dorsal fin, its greatest height about equal to greatest height of anal fin ; 

 a shallowly falcate anal fin; e^^es situated anterior to mouth, a line 

 drawn between the posterior margins of orbits passing anterior to sym- 

 physis of jaws (intersecting symphysis in larger specimens of S. tiburo) ; 

 a small orbit, separated from nares by a distance greater than horizon- 

 tal diameter of orbit; absence of an outer narial groove; a variable total 

 number of vertebrae, ranging in number from 142 to 202, mth those 

 over posterior part of coelomic cavity not enlarged; a small maximum 

 size, probably not exceeding 4 or 5 feet. Perhaps the maximum size 

 in some cases is even less (e.q., S. corona). 



Sphyrna (Platysqualus) tiburo (Linnaeus) 



Diagnosis. — -Characters mentioned in the diagnosis of the subgenus 

 Platysqualus are not repeated here. 



Sphyrna tiburo is unique among members of the genus Sphyrna in 

 having the head only slightly expanded, either spade or shovel shaped, 

 and measuring from 14.4 to 24.5 percent of TL in specimens up to 

 1,000 mm. long (higher values usually apply to smaller individuals 

 and/or to individuals of the subspecies vespertina). The anterior- 

 median pore patch on the underside of the head is also unique, the 

 patch characterized by a rather sharp, but smoothly rounded angle at 

 the posterior corner and by a long, narrow median posterior extension 

 (fig. 226). 



Sphyrna tiburo is also distinguished by the anterior margin of the 

 head being more or less evenly rounded between the ej^es, with neither 

 a median scallop nor narial depressions present; no inner narial groove; 

 origin of first dorsal fin distinctly posterior to axil of pectoral fin, a per- 

 pendicular line drawn ventrally from origin of dorsal intersecting pos- 

 terior part of pectoral fin; teeth not serrated, with short, weak, rather 

 blunt cusps. 



A comparison of Sphyrna tiburo with other members of the genus is 

 presented in table 1. 



Description.— Meristic data appear in table 6. Characters men- 

 tioned in the diagnoses of the subgenus Platysqualus and of S. tiburo 

 are not repeated here. 



Intranarial distance from one-tenth to one-sixth of TL (lower 

 values usually apply to larger individuals) ; outer posterior corners 

 of lateral expansions of liead broadly rounded and situated posterior 

 to corners of mouth; narial flap broad, blunt at tip, with the outer 

 edge curving gradually inward; length of snout about two-fifths 

 of head width; greatest transverse distance between corners of mouth 

 from one-third to a little more than two-fifths of greatest width 

 of head; sjTnphysis of jaws situated, in smaller individuals, slightly 



