40 



AMERICAN FISIIKS. 



D C 



^^SA 



In figure 2 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-opercidum^ fore-gill-cover, a, differs from the same 

 part, similarly marked, in No. 3, it being more rectilinear; while the 

 operculum J 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 sub-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 at page 45. 



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 situ- 

 ation 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 orbit of the eye ; while in that of the Bull Trout, Salmo 

 Erlox, it will run obliqu'ely very far below it. 



This distinction is very easy of observation, and is extremely im- 

 portant in the definition of species ; as indeed is everything connected 



