BRITISH FEESH-WATER FISH. 49 



characteristics which will he found appended to each. The 

 points upon which such minor distinctions depend, and 

 which will consequently be those to be particularly ob- 

 served and notedj are, — 



(1) The shape of the body, which can be best ascertained 

 by placing the fish straight on a sheet of white paper, and, 

 when it has stiffened, carefully tracing the outline with- a 

 pencil. 



(3) The relative position of the fins; and their form, 

 more especially as regards the back and tail fins * — whe- 

 ther forked, concave, square, or convex. To show the 

 importance of the latter point — the shape of the tail-fin — 

 it may be mentioned that upon this, next to the form of 

 the gill-covers, ichthyologists have relied in distinguishing 

 the Bull-trout from the true Salmon, — the tail-fin of the 

 former being convex, and that of the latter more or less 

 concave, in proportion to the age of the fish. 



(3) The form of the head and gill-covers -^j and their 

 length from the tip of the muzzle to the hinder margin, 

 as compared with that of the whole body, measuring from 

 the tip of the mxizzle to the extremity of the tail, where 

 it joins the tail-fin; and the relative position to the eye, 

 and to the lateral line, of a straight line drawn from the 

 upper front teeth to the lower posterior angle of the oper- 

 culum f . 



* Vide cut ahowing tlie names of tlie fins, page 32. 



t See page 30, and diagram of the various parts of the gill-coyer. 



X See cut at page 52. 



D 



