The True Sharks 543 



triangular serrate teeth, very massive in substance, and without 



denticles. As only the teeth are known, the actual relations 



of the several species of Corax 



are not certainly known, and 



they may belong to the Lam- 



nidcB. 



Family Sphymidae, or Ham- 

 mer-head Sharks. — The SpJiyrni- 

 d(B, or hammer -headed sharks, 

 are exactly like the Carcha- 

 riidcB except that the sides of 

 the head are produced, so as 

 to give it the shape of a ham- Fig. 336.— Teeth of Corax 



mer or of a kidney, the eye prlstodonius. 



being on the produced outer edge. The species are few, but 

 mostly widely distributed ; rather large, voracious sharks with 

 small sharp teeth. 



The true hammer-head, Sphyrna zygcena, Fig. 337, is common 

 from the Mediterranean to Cape Cod, California, Hawaii, and 

 Japan. The singular form of its head is one of the most ex- 

 traordinary modifications shown among fishes. The bonnet-head 

 {Sphyrna tihiiro) has the head kidney-shaped or crescent-shaped. 

 It is a smaller fish, but much the same in distribution and habits. 

 Intermediate forms occur, so that with all the actual dift'erences 

 we must place the SpJiyrnidoe all in one genus. Fossil hammer- 

 heads occur in the Miocene, but their teeth are scarcely different 

 from those of Carcharias. Sphyrna prisca, described by Agassiz, 

 is the primeval species. 



The Order of Tectospondyli. — The sharks and rays having no 

 anal fin and with the calcareous lamelte arranged in one or 

 more rings around a central axis constitute a natural group to 

 which, following Woodward, we may apply the name of Tecto- 

 spondyli. The Cyclospondyli {Squalida:, etc.) with one ring 

 only of calcareous lamellae may be included in this order, as 

 also the rays, which have tectospondylous vertebrae and differ 

 from the sharks as a group only in having the gill-openings 

 relegated to the lower side by the expansion of the pectoral 

 fins. The group of rays and Hasse's order of Cyclospondyli we 

 may consider each as a suborder of Tectospondyli. The origin 



