THE SALMON FAMILY. 



245 



the teeth, and (3) to the size and shape of the tail-fin, in 

 other words, by the principal organs of respiration, masti- 

 cation, and motion, which are admitted to be amongst the 

 most important and unvarying in piscine structure. 



First, differences in the form of the gill-covers*. These 

 differences afford by far the readiest and most certain test 

 for distinguishing between the species above named. They 

 are so obvious, that, upon comparing the gill-covers of 

 either of the fish with the figures here given, its identi- 



Salmon. 



Bull-Trout. 



Sea-Trout. 



fication must be a matter of ease. It may be remarked, 

 however, that the hinder margin of the whole gill-cover in 

 the true Salmon forms nearly a semicircle, whilst that of 

 the Bull-Trout approaches more closely to a right angle, 

 and both differ completely from the same parts in the gill- 

 cover of the Sea-Trout. The position and shape of the 

 several parts of which the gill-covers are composed also 

 vary considerably, as already pointed out at page 52. 



* The position of these is shown at page 30. They are often 

 erroneously called " gills," the real gills, however, being the red 

 spongy substance which they cover. 



