Morphology of the Fins 65 



their origin from a continuous fold of skin. In this species the 

 dorsal fins show much the same form. Other early sharks, con- 

 stituting the order of Acanthodei, have fins somewhat similar, 

 but each preceded by a stiff spine, which may be formed from 

 coalescent rays. 



Long after these appears another type of sharks represented 

 by Pleuracanthus and Cladodus, in which the pectoral fin is a 



FIG. 51. Cladoselache fyleri (Newberry), restored. Upper Devonian of Ohio 



(After Dean.) 



jointed organ fringed with rays arranged serially in one or two 

 rows. This form of fin has no resemblance to a fold of skin, 

 but accords better with Gegenbaur's theory that the pectoral 

 limb was at first a modified gill-arch. In the Coal Measures 

 are found also teeth of sharks (Orodontidce) which bear a 



FIG. 52. Fold-like pectoral and ventral fins of Cladoselache fyleri. (After Dean.) 



strong resemblance to still existing forms of the family of 

 Heterodontidtz, which originates in the Permian. The existing 

 Heterodontidce have the usual specialized form of shark-fin, with 

 three of the basal segments especially enlarged and placed side 



