528 



The True Sharks 



contracted than in the Asiastic species, called Heterodontus. 

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

 name of Gyropleurodus. 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 falcifer. 



Three families of Cestraciontes are recognized by Hay. The 

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



from the Lower Carboniferous, has the 

 teeth with a central crown, its surface 

 wrinkled. Of the Heterodontidce, Hybo- 

 dus, of the Carboniferous and Triassic, 

 is one of the earliest and largest genera, 

 characterized by elongate teeth of many 



FIG. 3i9.-Toothof Hybodus de- CUS P S > different in different parts of the 

 labechei Charlesworth. (After i aw somewhat as in the Hexanchida, 



Woodward.) 



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 Hexanchida and Hetero- 



FIG 320. Fin-spine of Hybodus basanus Egerton. Cretaceous. Family Hetero- 

 dontidce. (After Nicholson.) 



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

 genus two large hooked half -barbed dermal spines occur behind 

 each orbit. 



FIG. 321. Fin-spine of Hybodus reticulatus Agassiz. (After Zittel.) 



Palaospinax, with short stout spines and very large pectoral 

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

 by Woodward. Acrodus, from the Triassic, shows considerable 

 resemblance to Heterodontus. Its teeth are rounded and without 

 cusps. 



