The True Sharks 



529 



Most of these species belong to the Carboniferous, Triassic, 

 and Jurassic, although some fragments ascribed to Cestraciont 

 sharks occur in the Upper Silurian. Astera- 

 canthus, known only from fin-spines in the 

 Jura, probably belongs here. 



It is a singular fact first noted by Dr. 

 Hay, that with all the great variety of sharks, 

 ten families in the Carboniferous age, repre- 

 sentatives of but one family. Meter odontidcB, 

 are found in the Triassic. This family may 

 be the parent of all subsequent sharks and 

 rays, six families of these appearing in the 

 Jurassic and many more in the Cretaceous. 



Edestus and its Allies. — Certain monstrous 

 structures, hitherto thought to be fin-spines. 



Fig. 322. Fig. 323. 



Fig. 322. — Fin-spine of Hybodus canaliculatus Agassiz. 



Fig. 323. — Teeth of Cestraciont Sharks. (After Woodward.) a, Hybodus laevis 



Woodward (after Woodward); b, Heterodontus rugosus Agassiz; c, Hybodus 



delabechei Charlesworth. 



are now shown ^ / Dr. Eastman and others to be coalescent teeth 

 of Cestraciont sharks. 



These remarkable Ichthyodorulites are characteristic structures 



Fig. 324. — Edestus vorax Leidig, supposed to be a whorl of teeth. 

 (After Newberry.) 



of sharks of unknown nature, but probably related to the 

 Heterodontidce . Of these the principal genera are Edestus, 

 Helicoprion, and Campylo prion. Karpinsky regards these 

 ornate serrated spiral structures as whorls of unshed teeth 



