Animals Before Man 



has a spine in front of each of the two back 

 fins. The fossil species not only had spines on 

 their back fins, but on their side fins as well, 

 after the fashion we find so effective in the cat- 

 fishes, and in some species the side fins are even 

 represented by a series of spines. These are 

 usually ornamented with ridges, but sometimes 

 quite elaborately decorated with raised orna- 

 mental figures. 



The skeletons of these sharks were mostly 

 cartilaginous and consequently not preserved, 

 so that most of them are known only from 

 their teeth and the spines with which their fins 



were armed 

 a circumstance 

 that has caused 

 much trouble, 

 for as these are 

 rarely found 

 together, one 

 name has often 



been given to the teeth and another to the 

 spines of the same individual, thus making two 

 species out of one. Worse than this, teeth 



108 



Teeth of Cladodus, a Carboniferous shark, 

 showing the different kinds of teeth in 

 one shark. Somewhat enlarged. (Af- 

 ter Dean.) 



