528 



The True Sharks 



contracted than in the Asiastic species, called Heterodontus. 

 For this reason Dr. Gill has separated the former iinder the 

 name of Gyropleiirodits. The differences are, however, of slight 

 value. The genus Heterodontus first appears in the Jurassic, 

 where a number of species are known, one of the earliest 

 being Heterodontus jalcifer. 



Three families of Cestraciontes are recognized by Hay. The 

 most primitive of these is the group of Orodontidce. Orodus, 



from the Lower Carboniferous, has the 



teeth with a central crown, its surface 



wrinkled. Of the Heterodontidce, Hyho- 



dus, of the Carboniferous and Triassic, 



is one of the earliest and largest genera, 



characterized by elongate teeth of many 



Fig. 3i9.-Toothof H„bodusde-C^^V^' different in different parts of the 



labechei Charlesworth. (After jaw, somewhat as in the HexanchidcF, 



Woodward.) , ,. • , •, ■ i 



the median points being, however, 



always longest. The dorsal fins are provided with long spines 

 serrated behind. The vertebrae with persistent notochord show 

 qualities intermediate between those of Hexancliidm and Hetero- 



FiG 320. — Fin-spine of Hybodus basaniis Egerton. Cretaceous. Familj' Hetero- 

 dontidce. (After Nicholson.) 



doiitidcc, and the same relation is shown by the teeth. In this 

 genus two large hooked half -barbed dermal spines occur behind 

 each orbit. 



Fir ^21 — Fin-spme ot Hiibodu\ uticidatiis Agassiz (After Zittel.) 



Pahcospiiiax, with short stout spines and very large pectoral 

 fins, formerly regarded as a dogfish, is placed near Hcterodoiitus 

 by Woodward. Acrodus, from the Triassic, shows considerable 

 resemblance to Heterodontus. Its teeth are rounded and without 

 cusps. 



