Tlie Coming of Vertebrates and Rise of Fishes 



anything or anybody to pieces with their many 

 rows of sharp and wicked-looking teeth, when a 

 large proportion of the sharks are under 6 feet 

 in length, and some of the very largest have 

 small, feeble teeth, and are quite harmless so 

 far as man is concerned. And only one of 

 the many rows of teeth is in use at one time, 

 the others forming a reserve supply, to be 

 drawn upon in case of accident, and, like re- 

 serves of soldiers, these are kept at the rear and 

 lie down out of the way. The sharks of the 

 Carboniferous seas were mostly of small size, 

 from 3 to 6 feet in length, and their teeth were 

 fitted for holding or crushing rather than for 

 cutting purposes. In some the teeth were ar- 



Port Jackson shark, Cestracion phiUppi. 



ranged like so many little cobblestones, and 

 these species have received the appropriate 

 name of pavement-toothed. But one genus of 

 8 105 



