334 



The Descent of Man. 



PaktU 



protect the jaws, when one male charges another with wonderful 

 violence ; but the greatly developed teeth of the male American 

 salmon may be compared with the tusks of many male mammals, 

 and they indicate an oifensive rather than a protective purpose. 



Fig 28 



Head of female salmon. 



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

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

 thornback (^Eaia clavata) the adult male has sharp, pointed 

 teeth, directed backwards, whilst those of the female are broad 

 nnd flat, and form a pavement ; so that these teeth differ in the 

 two sexes of the same species more than is usual in distinct 

 jrenera of the same family. The teeth of the male become sharp 

 'in'y when he is adult- whilst young they are broad and fia,t 



