OUTLINES OF ICHTHYOLOGY. 183 



without exception belonging to the Cyprinidae, or 

 Carp family. 



To decide with certainty between different 

 genera, or different species of the same genus, re- 

 quires a more minute observation and comparison, 

 according to the " characteristics " which will be 

 found appended to each. The points upon which 

 such minor distinctions depend, and which will 

 consequently be those to be particularly observed 

 and noted, are : — 



(a.) 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. 

 (b.) The relative positions of the yfw (see p. 170); and their 

 form, more especially as regards the back- and tail-fins, whether 

 forked, concave, square, or convex, (c.) The form of the head 

 z.nA gill-covers (see p. 16S), 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 muzzle to the 

 extremity of the tail, (d.) The nature of the tcctk and the 

 names, if possible, of the boius on which they are placed 

 (see pp. 171-174). (e.) The colouring of the body and fins 

 of the fish, when first caught, (f.) The number of scales 

 forming the lateral line. (G.) The number of 7?«-;i?)'^, or sup- 

 ports, in each fin. 



These, in the tail fin, are counted from the first long ray out- 

 side, either above or below; and in the other fins in all cases 

 from the first ray nearest the head of the fish. 



The fin-rays are denoted, as in these pages, by placing the 

 initial letter of the name of the fin before the numeral express- 

 ing the number of rays it contains — as D. (for " Dorsal ") 9 : P. 

 (for "Pectoral") 15 : &c. Take, for example, the fin-rays of 

 the Gudgeon, which are particularly simple : — 



D. 9: P. 15 : V. 8: A. 7: C. igj. 



Occasionally there are short incumbent rays on the base of 



