IV 

 THE SHARKS 



ALL true fishes may conveniently be grouped into the 

 four sub-classes that have been noted (p. 8) in the introduc- 

 tory chapter. These are now in turn to be considered, and 

 in this review the principal forms, fossil and recent, of each 

 group must be exemplified. From the standpoint of their 

 structural and developmental characters, a general idea of 

 the mutual relationships of the fishes may finally be 

 deduced. 



The sub-class Elasmobranchii, which includes the sharks 

 and rays, is usually regarded as representing most nearly 

 the persistent ancestral condition of fishes, and, indeed, of 

 all other jaw-bearing vertebrates. As a group it should 

 certainly be taken first in the present discussion, as a con- 

 venient basis of comparison. 



Sharks and rays should be looked upon at the beginning 

 as the representatives of the oldest, most widely diffused, 

 and possibly largest group of fishes. In their living 

 forms they suggest but faintly the number and variety of 

 their fossil kindred. It is generally thought that the his- 

 tory of this group, when more perfectly determined, is to 

 furnish the most important evidence as to the general 

 lines of descent of the fishes. 



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