50 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 119 



Individuals gray or grayish brown dorsally, becoming progres- 

 sively more pale below, and often with small dark spots on side of 

 body; occasional spots on caudal fin. 



Variation. — Specimens of Sphyrna tiburo from the Pacific usually 

 have more pointed heads than specimens from the Atlantic (figs. 12a, b) . 

 This difference is also reflected in proportional measurements, the 

 relative width of the head and transverse distance between the nares 

 averaging slightly greater for specimens from the Pacific (table 6 and 

 fig. 13). As a result, Springer (1940a, pp. 161, 162) described the 

 Pacific population as a new species, Sphyrna vespertina, although 

 Bigelow and Schroeder (1948, p. 428) later placed it in the synonymy 

 of S. tiburo. 



For several reasons the two populations are regarded here as having 

 differentiated only to the subspecies level. First, no differences com- 

 parable to those found for head shape were discovered for other 

 morphometric characters (table 6). In addition, some individuals are 

 intermediate in head shape (fig. 12c). As mentioned previously, such 

 specimens possibly may have resulted from a partial reamalgamation 

 of the populations following initial differentiation of the two forms. 



Vertebral counts were made on four specimens of S. tiburo tiburo 

 and three of S. tiburo vespertina. These counts, which range from 163 

 to 172 for the latter form and from 142 to 170 for the former, are 

 interesting both because of the wide range in variation found in the 

 typical subspecies and also because of the close agreement in vertebral 

 number between two of the individuals of S. tiburo tiburo and the three 

 specimens of S. tiburo vespertina. The two specimens of S. tiburo 

 tiburo with the lowest counts (142 and 143) are from the eastern Gulf 

 of Mexico, while those with the higher coimts (159 and 170) are from 

 Rhode Island and Brazil. (It should be mentioned here that these 

 differences are not attributable solely to variation in number of caudal 

 vertebrae.) Although it is possible that larger series of counts will 

 show this apparent difference to be clinal in nature, a study of vertebral 

 variation in sharks, recently completed by Drs. V. G. Springer and 

 J. A. F. Garrick (1964) indicates the existence of a number of closely 

 related species differing only in vertebral counts. This suggests that 

 those specimens of S. tiburo tiburo from the eastern Gulf of Mexico 

 represent a sibling form that may be distinguished from other pop- 

 ulations of the same subspecies only by number of vertebrae. 



Range. — Despite statements to the contraiy (Bigelow and Schroe- 

 der, 1948, pp. 424-425; Herre, 1953, p. 27; Briggs, 1960, p. 173; and 

 1961, p. 552), there appear to be no substantiated records of Sphyrna 

 tiburo outside of the Western Hemisphere. 



Sphyrna tiburo tiburo is common in shallow, inshore waters of the 

 western Atlantic, where it ranges from southern Brazil northward. 



