CLASSIFICATION BY GILL-COVERS. 



49 



In figure 3 of this cut, representing the gill-cover of the true 

 Salmon, it will strike any casual observer that the hinder 



margin of the whole covering forms nearly a semicircle, while 

 that of No. 3, the Bull Trout, approaches more nearly to a 

 rectangular figure. In the former, the pre-operculum, the fore- 

 gill-cover, A, differs from the same part, similarh^ marked, in 

 No. 3, it being more rectilinear ; while the operculum, the gill- 

 cover proper, b, of the former slopes hindward and backward ; 

 the same portion, b, in No. 3, cutting in a horizontal line upon 

 the joints of the suh-operculum and inter -operculum. 



And in all respects both differ entirely from the arrangement 

 of the same parts in the head of the Silver Trout, exhibited in 

 the cut last preceding, page 48. 



The most striking consequence of these differences is, that 

 a straight line, drawn backward from the front teeth of the 

 upper jaw, the mouth being closed, to the longest posterior 

 projection of the gill-cover, will, in the three fish, run at a 

 totally different angle to the horizontal line of the body ; and 

 will occupy an entirely different situation in respect to the 

 eye ; such a line in the head of the Salmon [Salmo Salar), and 

 in the Silver Trout {Salmo Lacustris), passing close below the 



