53 



The True Sharks 



cemented togther and extending outside the mouth, "sharp, 

 piercing teeth which were never shed but became fused in 

 whorls as the animals grew." 



Dr. Eastman has, however, shown that these supposed 

 teeth of Edestus are much like those of the Cochliodontida, and 

 the animals which bore them should doubtless find their place 



FIG. 325. Helicoprion bessonowi Karpinsky. Teeth from the Permian of 

 Krasnonfimsk, Russia. (After Karpinsky.) 



among the Cestraciont sharks, perhaps within the family of 

 Heterodontidce. 



Onchus. The name Onchus was applied byAgassiz to small 

 laterally compressed spines, their sides ornamented with smooth 

 or faintly crenulated longitudinal ridges, and with no denti- 

 cles behind. Very likely these belonged to extinct Cestraciont 

 sharks. Onchus murchisoni and Onchus tenuistriatus occur in 

 the Upper Silurian rocks of England, in the lowest strata in 

 which sharks have been found. 



To a hypothetical group of primitive sharks Dr. Hassc 

 has given the name of Polyospondyli. In these supposed 



