130 



THE CHAIN OF LIFE. 



l)rcpondcrancc of those species with flat, crushing teeth fitted 

 for grinding shells,* which in diminishing numbers continue up 

 to the present time, when they are represented by the Port 

 Jackson Shark and a few other species. The increase toward 

 the modern time of the true Sharks =* with sharp cutting teeth, is 

 obviously related to the increase of the ordinary fishes which 



i^ 



Fig. 117. — Teath and Spines of Carboniferous Sharks. Nova Scotiu. 



(t, D I f>lodus penetrans 2mA D. acinaces. b, Psamwodus. c, Ctenofitychius c*-t.sitifi/\\ 

 d, Spine, GyraaiHthus magnijicus. One-eightli natural nizs.^'Acadian Geology. 



furnish them with food. Another curious difference, connected 

 probably with the same circumstance, is the fact that in the 

 sharp-toothed Sharks of the Carboniferous the two side fangs 

 of each tooth are the largest, or are exclusively developed 

 (Fig. 117, a), while in later periods the central point becomes 



' Cesh'acionts. • Selachians. 



