NO. 3539 HAMMERHEAD SHARKS GILBERT 3 



indentation on the median anterior margin of the head, while in 

 others this indentation is lacking. The presence or absence of an 

 outer narial and/or inner narial groove and the relative length of the 

 latter help to distinguish even the smallest individuals of a particular 

 species. 



The various chondrocrania also are distinctive. Shape of both 

 the anterior and posterior parts of the preorbital process, degree of 

 development of the "wmgs" on the olfactory cartilage, development 

 of the rostral "wings," shape of the rostral cartilage, presence or 

 absence of accessory rostral cartilages, presence or absence of a 

 rostral fenestra, and shape of the anterior fontanelle all differ from 

 species to species. 



The arrangement of the Ampullae of Lorenzini on the ventral 

 surface of the head is relatively constant for each species, with the 

 arrangement of those in the anterior-median area being particularly 

 diagnostic. These ampullae, which are long canals filled with mucus, 

 comprise a highly modified part of the lateral-line system, and it has 

 been suggested that they may be involved in the detection of temper- 

 ature changes, hydrodynamic pressure changes, tactile stimulation, 

 and/or electric gradients (Young, 1962, p. 172). For brevity, these 

 structures are referred to as mucal pores throughout this paper. 



Teeth vary according to species, although in several instances the 

 differences are rather slight. Shape and arrangement of the teeth, 

 as well as the presence or absence of serrations, are distinctive. 



There are differences between the species in size and shape of the 

 fins and in the positions of these structures relative to each other. 

 The positions of the foiu"th and fifth gill slits in relation to the insertion 

 of the pectoral fin and the relative lengths of the gill slits are important. 

 Finally, the various species differ in the presence or absence of a 

 lower precaudal pit and the shape of the upper precaudal pit. 



Recent studies by Drs. Victor G. Springer and J. A. F. Garrick 

 (1964) on carcharhinid sharks indicate that the natiu"e and nimiber of 

 the vertebrae can be an extremely useful taxonomic tool, often showing 

 differences not evident from the study of external morphology. Un- 

 fortunately, the value of vertebral characters was not recognized 

 while my study was in progress, and consequently only a few counts 

 were made for the species of Sphyrna. 



Other characters that are probably of taxonomic importance in 

 hammerheads are the morphology of the pelvic claspers and the 

 electrophoretic patterns of the serum proteins. The latter were not 

 studied because of the unavailability of fresh specimens and lack of 

 facilities. Because of the relatively small size and immaturity of 

 the available specimens, claspers of only three species, Sphyrna tudes, 

 S. media, and S. tiburo, were examined. 



