NO. 3539 HAMMERHEAD SHARKS GILBERT 33 



USNM 51289 (head), Wakanoura; USNM 51291 (head), Nagasaki; 

 FAINH 57423 (1), Osaka market; AICZ 1042 (1), Yenoshima; SU 

 24125 (1), Misaki; UMMZ 179078 (1), vicinity of Choshi; UMMZ 

 179079 (1), Hakata Bay, northern Kyushu; UMMZ 179080 (1), 

 Suruga Bay. Mexico: USNM 190592 (10), Gulf of California, 

 southern end of Angel de la Guarda Island; UMMZ 61032 (1), Cedros 

 Island. New Zealand: UMML imcataloged (2), Bay of Islands, 

 North Island. Panama: MCZ 515 (1), no fiu-ther data. Peru: 

 USNM 77711 (head), I. Lobos de Tierra; MCZ 421 (1), MCZ 441 

 (3), Magdalena Bay; MCZ 382 (2), Payta; SIO 58-29 (1), "off Peru"; 

 SU 13003 (1), no further data. United States (CaHfornia): SIO 

 58-224, SIO 58-382 (jaws), off La Jolla, San Diego County; LACM 

 uncataloged (jaws and chondrocranium), oft' San Diego. 



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

 genus Sphyrna are not repeated here. 



Sphyrna zygaena is characterized (as is S. lewini, its closest relative) 

 by a long second dorsal fin lobe and a low second dorsal fin, the length 

 of the former about twice greatest height of fin. The anterior- 

 median pore patch on the imderside of the head is unique, the patch 

 characterized by a broad, smoothly rounded angle at the outer- 

 posterior corner and a broad, pointed posterior extension in the 

 median -posterior area (fig. 22c). 



S. zygaena is distinguished from S. lewini by lack of a median 

 indentation (scallop) on anterior margin of head; a longer inner 

 narial groove, which extends at least 50 percent of distance from 

 inner margin of nares to tip of snout; orbit closer to nares, being 

 separated by a distance equal to about half of greatest horizontal 

 diameter of eye; tip of second dorsal lobe not reachmg as near to 

 upper precaudal pit, the lobe extending, at most, about two-thirds 

 of distance from second dorsal base to pit; a slightly more deeply 

 falcate anal fin, the deepest part of the notch on the distal margin 

 situated slightly posterior to a line drawn perpendicularly from the 

 posterior axil of the fin; a shorter anal base, the length of which is 

 slightly less than length of pectoral base; a more acutely pointed 

 posterior ridge to the upper precaudal pit (fig. 21c); absence of a 

 lower precaudal pit; absence of a rostral fenestra; and several other 

 chondrocranial characters (pi. 6c). 



A comparison of Sphyrna zygaena with other members of the genus 

 Sphyrna is presented in table 1 . 



Description. — Meristic data appear in table 4. Characters men- 

 tioned in the diagnoses of the subgenus Sphyrna and of S. zygaena 

 are not repeated here. 



Head moderately expanded, the greatest width from 26.1 to 29.0 

 percent of TL (in specimens less than 1,000 mm. long); shallow 



219-937—67—3 



