16 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 119 



Oils, and there are substantiated records of fatal attacks on humans 

 (Gudger, 1937, pp. 417, 418). 



Hammerheads along the eastern North American coast are known 

 to migrate northward in summer and southward in winter, often 

 traveling in schools in both inshore and offshore waters (Bigelow and 

 Schroeder, 1948, pp. 442-444). To the north, Sphyrna zygaena is the 

 only species involved in such migrations, whereas, to the south, S. 

 lewini is the predominant species. The fact that the only specimens of 

 Sphyrna zygaena seen by me from south of the Carolinas (UMML 

 5639 and UMML 10444, from Miami, Fla.) were taken in winter 

 seems to confirm this. Presumably similar migrations occur in other 

 parts of the world. 



Although S. media and S. corona have been taken in the same areas 

 from Panama Bay north to southern Mexico (map 5), in only one of 

 the collections examined (from Chiapas, Mexico) were the two ac- 

 tually taken together. Whether this is due to a partial ecological 

 separation or simply to lack of material is presently not known. 



The number of young in a brood appears to be directly proportional 

 both to size of the individual and to maximum size of the species. 

 The smallest recorded brood (8) was found in S. tiburo, the largest 

 (38) in S. mokarran. 



Zoogeography. — The present distribution of the hammerhead 

 sharks indicates, on the whole, a slow rate of evolution. The members 

 of the subgenus Sphyrna (S. zygaena, S. lewini, S. couardi, and *S'. 

 mokarran) apparently have undergone no intraspeciiic differentiation, 

 despite worldwide distributions (for three of the four species) and 

 long periods of separation of the various segments of their respective 

 populations. The five forms included within the subgenus Platy- 

 squalus (S. tildes, S. corona, S. media, S. tiburo tiburo, and S. tiburo 

 vespertina) are morphologically similar to one another, and thus pre- 

 sumably have undergone a more recent evolution. Only in the case 

 of S. tiburo is it possible to estimate how long it has taken for evolution 

 to have occurred, because the two subspecies probably differentiated 

 following the most recent (Pliocene) emergence of the Central Ameri- 

 can isthmus. However, the presence of individuals of this species 

 with a head shape more or less intermediate between typical tiburo 

 and vespertina indicates that the above explanation may not be as 

 simple as first appears. Possibly the initial separation goes farther 

 back and the intermediate examples have resulted from a partial 

 reamalgamation of the stocks. 



Insufficient evidence exists to explain the present distribution of S. 

 tades, S. corona, and S. media. Evolution of these three species is 

 almost certainly correlated Avith the various submergences and emer- 

 gences of the Central American land mass, although it is impossible 



