52 



THE ANGLEE-NATURALIST. 



type (in so far at least as regards those externals which 

 most readily strike an uneducated eye), the peculiarities of 

 structure of the gill- cover, apparently of little moment, 

 may, from their importance and permanence, not unfre- 

 quently indicate totally distinct and immutable species. V 



Almost all the really distinct species of the Salmonidte 

 are distinguished from one another principally by the 

 form of the head and the formation of the gills, in the first 

 degree, and by the dental system in the second, any per- 

 manent and unvarying difference in these, coupled with 

 variations of colour, form, habit, or the like, which might 

 otherwise be deemed casual, being held to constitute a 

 distinct species. 



An example of this difference in the form of the gill- 

 cover will be seen in the annexed engraving, in which the 

 right-hand figure represents the gill- cover of the Sea- or 

 Salmon-Trout, the central one that of the Bull-Trout, and 

 the figure on the left hand that of the True Salmon. 



Salmon. 



Bull-Trout. 



Sea-Trout. 



On comparing these figures, it will at once be evident 

 that the hinder margin of the whole gill- cover in the true 



