FISHES. 379 



pared witli the tusks of many male mammals, aiid they 

 indicate an offensive rather than a protective purpose. 



The salmon is not the only fish in which the teeth differ 

 in the two sexes; as this is the case with many rays. In 



Fig. 28. Head of female salmon. 



the thornback {Raia Clavata) the adult male has sharp, 

 pointed teeth, directed backward, while those of the female 

 are broad and flat, and form a pavement; so that these 

 teeth differ in the two sexes of the same species more than 

 is usual in distinct genera of the same family. The teeth 

 of the male become sharp only when he is adult; while 

 young they are broad and flat like those of the female. As 



