;6o 



FISHES 



characters whose variations have been studied in the plaice, 

 are the number of the tubercles on the head, the number of 

 gill-rakers on the first branchial arch, the breadth of the body 

 and length of the head in proportion to the total length, and 

 the degree of spinulation of the scales. The number of tubercles 

 on the head in the great majority of specimens is five, occasion- 



66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 



Fig. 29. — Variation in number of dorsal fin-rays in a sample of North-Sea 

 plaice. 



ally more or fewer occur. The presence of minute spines on the 

 scales is a secondary sexual character usually confined to mature 

 males : when least developed, it is present only on the fin-rays ; 

 in other cases it occurs also on the head, in others extends to 

 the body as well. By thus studying the range of variation for 

 several characters, it is possible to distinguish local forms or 

 varieties when single individuals cannot be distinguished. Thus 

 there are indications that the number of fin-rays is less and 



