BKITISH FRESH-WATER FISH. 



53 



Salmon forms nearly a semicircle, whilst that of the Bull- 

 Trout approaches more nearly a right angle, and both 

 differ entirely from the same parts in the gill-cover of the 

 Sea-Trout. A remarkable result of these differences is, 

 that a straight line drawn backwards from the front-teeth 

 of the upper jaw (the mouth being closed) to the lower 

 posterior angle of the operculum will, in the three fishes, run 

 at a totally different angle to the lateral line of the body, 

 and will occupy an entirely different position in respect of 

 the eye. There is likewise a difference in the arrangement 

 of the teeth. 



TBISTOMA COCCINEUM. 



A PARASITE SOMETIMES ATTACHED TO SEA-FISH. 



