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 Cladodiis, in which the pectoral fin is a 



Fig. 51. — Cladoselache fyleri (^fewbe^ry), 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 Cladondache jyhri. (XHer Dean.) 



Strong resemblance to still existing forms of the family of 

 HeterodontidcB, which originates in the Permian. The existing 

 Heterodontida; have the usual specialized form of shark-fin, with 

 three of the basal segments especially enlarged and placed side 



