160 ORDERS OF FISHES. 



'/nodus, the teeth have the form of broad crushing plates, 

 adapted for the comminution of Molluscs or Crustaceans. 

 In fact, in these forms the teeth very closely resemble 

 those of the living Port Jackson Shark {Cestracion). In 

 the second type, as in Cladodus, Orodus, and Glossodus, the 

 teeth are of what is called the " Hybodont " form, having a 

 general conical shape, and consisting of a central principal 

 cone, flanked by smaller secondary cones, A third group 

 may be constituted for teeth of the type of Pdcdodus (fig. 

 522, 4), in which the teeth are concentrically wrinkled 

 round their bases, transversely elongated, with a compressed 

 petal-shaped expansion above, the summit of which forms a 

 serrated cutting edge. The " Petalodonts " are characteristic 

 of the Carboniferous rocks. 



In the Permian series the remains of Cestraphori are 

 scanty, but they are very numerous in all the great forma- 

 tions of the Secondary period. The four most important 

 Mesozoic genera are Hyhodus, Acrodus, Strophodtis and Pty- 

 chodus. 



In the genus Hyhodus (fig. 524) the teeth are shark-like, 

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



Fi.u'. 5-24.— Tooth Fig. 52.j.— Fiii-sipiiic (if i/(//«j<?«,s. CietaL-euiis. 



<if Hyhvdus. 



They consist of a central " principal " cone, with smaller 

 "secondary" cones on each side. The fin-spines (fig. 525) 

 in this genus are longitudinally grooved, and carry a series 

 of small teeth on their hinder or concave margin. Species of 

 Hyhodus abound in the Triassic and Jurassic formations, and 

 occur, though less abundantly, in the Cretaceous rocks. 



In the genus Acrodus (fig. 526) the teeth are more like 

 those of the Port Jackson Shark. The front teeth are pointed 

 and resemble those of the Hybodonts, but the back teeth are 

 adapted for crushing shell-fish. Each of these crushing teeth 



