ELASMOBRANCHII. 



339 



macanthus, Oracanthus, Onchus, Leptacanthus, and Edestes. The 

 fossil teeth of the Carboniferous Rocks have also been referred 

 to many genera, of which the more important are Cochliodus 

 (fig. 297), Psamtnodus, Orodus, Petalodus (fig. 296, 4), Ctenop- 

 tychius (fig. 296, 6), Cladodus, 

 Centrodits, Glossodus, and Petro- 

 dus. Two types may be distin- 

 guished in these teeth. In one 

 type, as in Cochliodus (fig. 297) 

 or Psammodus, the teeth have 

 the form of broad crushing 

 plates, adapted for the com- 

 minution of Molluscs or Crus- 

 taceans. In fact, in these forms 

 the teeth very closely resemble 

 those of the living Port Jackson 

 Shark (Cestracion}. In the other type, as in Cladodus, Orodtts, 

 and Glossodus, the teeth are of what is called the " Hybodont " 

 form, having a general conical shape, and consisting of a cen- 

 tral principal cone, flanked by smaller secondary cones. 



In the Permian series the remains of Cestraphori are scanty, 

 but they are very numerous in all the great formations of the 

 Secondary period. The four most important Mesozoic genera 

 are Hybodus, Acrodus, Strophodus, and Ptychodus. The almost 

 exclusively Triassic genus Ceratodus has generally been referred 

 here also, but its true affinities are with the Dipnoi. 



In the genus Hybodtis (fig. 298) the teeth are shark-like, but 

 are not so trenchant as they are in the true Sharks. They 



Fig. 297. Dental plates of Cochliodus 

 ntortus. Mountain Limestone (Carbon- 



Fig. 299. Fin-spine of Hybodus. Cretaceous 



consist of a central " principal " cone, with smaller " second- 

 ary" cones on each side. The fin-spines (fig. 299) in this 

 genus are longitudinally grooved, and cairy a series of small 

 teeth on their hinder or concave margin. Species of Hybodus 

 abound in the Triassic and Jurassic formations, and occur, 

 though less abundantly, in the Cretaceous Rocks. 



In the genus Acrodus (fig. 300) the teeth are more like those 

 of the Port Jackson Shark. The front teeth are pointed, and 



