OUTLINES OF ICHTHYOLOGY. 185 



eye), the peculiarities of structure of the gill-cover, 

 apparently of little moment, may, from their im- 

 portance and permanence, not unfrequently indi- 

 cate totally distinct and immutable species. 



Any permanent and unvarying difference in the 

 form of the head, formation of the gill-covers, and 

 dental system, coupled with variations of colour, 

 form, habit, or the like, which might otherwise be 

 deemed casual, are held to constitute a distinct 

 species. 



A good example of this difference is seen in the 

 form of the gill-covers of the Salmon and the Sea 

 Trout the hinder margin of the gill-cover forming 

 in the true Salmon almost a semicircle, whilst that 

 of the Sea Trout approaches more nearly a right 

 angle, and both differ somewhat from the gill- 

 covers of the Bull Trout. 



