The Coming of Vertebrates and Rise of Fishes 



that of shark 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 be- 

 long in tlie same 

 category as bones, 

 intimately as the 

 two are connect- 

 ed in mammals. 



Many of the 

 early sharks had 

 the fins armed 

 with a sharp, 

 strong spine, and 

 although fin 

 spines have gone 

 out of fashion 

 among sharks, 

 there are still a 

 few species that 

 preserve the fam- 

 ily traditions, 



such as the Port Jackson shark, just referred 

 to, or the more familiar spiny dogfish, which 



107 



Spine from the back fin of a Carbon- 

 iferous shark, and one from the side 

 fin of an Amazonian catfish. 



