Fishes of the Western North Atlantic 409 



Sfecies. It was long thought that this was a very monotonous genus Including some 

 four or five species at most the world over. However, recent studies have shov/n that 

 the western Atlantic alone actually supports at least five well marked representatives, 

 separated by well defined and easily detectable characters, but which are so overshadowed 

 by the bizarre appearance of the head that they were largely overlooked in most of the 

 early accounts of the genus. Three new species have been described recently from the 

 eastern tropical Pacific also,* while the remaining sphyrnids of the Indo-Pacific region 

 as a whole stand in urgent need of critical revision. Unfortunately many of the older 

 descriptions, other than those of S. tiburo, which is the most easily recognizable member 

 of the genus in the Atlantic, omit precisely those characters that have recently been found 

 to be specific. Hence there is no knowing to which species, as now recognized, they actually 

 referred. The case is still further complicated by the fact that opportunity has not yet 

 been offered for a sufficiently extensive comparison of the species now known to exist in the 

 Atlantic with those of the Pacific and Indian Oceans. Consequently, the accompanying 

 Key is restricted to Atlantic species. Fortunately it is clear to which of the Atlantic forms 

 the Linnaean name zygaena (type species of the genus) actually referred, because 

 WillughbyV illustration of the lower surface of his zygaena, to which Linnaeus refers as 

 one of the bases of the species, is an excellent representation of a Hammerhead with head 

 rounded in front, with eyes close to corners of oculo-nasal prominences, and with long 

 caudal peduncle. 



Key to Atlantic Species 



I a. Anterior contour of midsector of head evenly rounded or nearly straight; not in- 

 dented or scalloped in median line (Figs. 82 B, 86 A). 



2a. Contour of head only slightly concave opposite nostrils, if at all (Fig. 82 B) 3 

 groove from nostril, if any, shorter than horizontal diameter of eye; free tip of 

 2nd dorsal not longer than its anterior margin; posterior margin of anal only 

 weakly concave; teeth near corners of mouth rounded, without cusps. 



tiburo Linnaeus, 1758, p. 420. 

 2b. Contour of head deeply scalloped opposite nostrils (Fig. 86 A); grooves from 

 nostrils more than twice as long as horizontal diameter of eye; free tip of 2nd 

 dorsal considerably longer than its anterior margin; posterior margin of anal 

 deeply concave; teeth near corners of mouth like those further forward, with 

 cusps. zygaena Linnaeus, 1758, p. 436. 



lb. Anterior contour of head unmistakably indented or scalloped in midline. 



3a. Free tip of 2nd dorsal only about as long as its vertical height, and considerably 

 shorter than its anterior margin; teeth serrate on cusps as well as basally. 



/«i^j Valenciennes, 1822, p. 428. 



4. Vesfertlna, media and corona Springer (Stanford Ichth. Bull., i [5], 1940: 161-169). 



5. Hist. Pise, 1686: pi. B, I. 



