316 PECULIARITIES OF PLAICE FROM DIFFERENT FISHING GROUNDS. 



for his kindness and courtesy in providing me with facilities for 

 carrying out this part of the work in the Natural History Museum 

 at South Kensington. I will describe first the characters of the 

 samples of plaice from the North Sea and the South-west coast. 



I. The Variations Observed in Plaice from different parts of 

 THE North Sea and Channel. 



The method I have employed is not exactly the same as that adopted 

 by Duncker, who followed the example set by Heincke in his papers on 

 the varieties of herrings, in the publications of the Commission zur 

 Untersuchung der Deutschen Meere. The first step in either method 

 is the actual observation by measurement and counting of the principal 

 characters in eoch individual fish. The characters selected by Duncker 

 were the following : — 



1. The number of vertebrae in the caudal peduncle. 



2. The number of vertebra} between the abdomen and the caudal 

 peduncle. 



3. The number of abdominal vertebra?. 



4. The total number of vertebrae. 



5. The number of gill-rakers on the first branchial arch. 



6. The number of dorsal fin-rays. 



7. The number of ventral fin-rays. 



8. The length of the caudal peduncle. 



9. The mean height of the same. 



10. The greatest height of the body without the marginal, or dorsal 

 and ventral, fins. 



11. The greatest length of the head, on the upper side. 



12. The extent of spinulation in the males. 



In accordance with Heincke's method Duncker divided the variations 

 observed in each of these characters into stages, which he denoted by 

 symbols. The object of this is, firstly, to eliminate small errors of 

 observation; secondly, to allow the differences to become more distinct; 

 and thirdly, to show the relations between different variations ; that is, 

 to exhibit what is known as correlation. The character of each fish 

 is thus represented by a formula consisting of a number of symbols, 

 and a difference between local races may be demonstrated by the fact 

 that the most frequent formulas are different in the two cases. 



I decided to neglect the examination of the number of vertebra? 

 altogether. The variations in these were too small to be likely to 

 exhibit any differences between the different samples I was investigating, 

 and so far as they existed would be sufliciontly represented by the 

 variations in the number of fin-rays and length of the caudal peduncle. 



