1V 
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 — q 
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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