THE DEVONIAN PERIOD 



463 



The teeth occurred in clusters (Fig. 440) and were arranged in six 

 or seven rows one behind the other. The fins were very simple, con- 

 sisting of a flap of skin strengthened by straight rods of cartilage. 

 It was very unlike modern sharks in the contour of its body. 



Fig. 439. — A y shark, Cladoselache,- which sometimes reached a length of six feet (see 

 Fig. 440) ; B, the Port Jackson shark, Cestracion, a modern shark of ancient type. 



Other sharks, now represented by the Port Jackson shark of Aus- 

 tralian waters (Cestracion, Fig. 439 B), were abundant in the later 

 Paleozoic, judging from the number of their spines and pavement 

 teeth. The teeth of these sharks have been called " cobblestone " 

 pavement teeth because of their resemblance in shape and arrange- 

 ment in the jaw to a pavement. Such teeth would be of use in crush- 

 ing thin-shelled crustaceans 

 and shellfish, but could not 

 have been used for rending. 

 It is evident, therefore, that 

 their possessors probably lived 

 on muddy bottoms and fed 

 on brachiopods, pelecypods, 

 or crustaceans. The structure of the tooth plate of sharks " is very 

 much like that of a shark's skin, and it is the teeth of these and other 

 sharks that best illustrate the fact that teeth are really modifications 

 of the skin and do not belong in the same category as bones." 



CLELAND GEOL. — $C 



Fig. 440. — Teeth of Cladoselache. 



