280 THE VARIATION, RACES AND MIGRATIONS OF THE MACKEREL. 



their primitive cliaracter. On this account, in spite of the slender 

 basis for drawing racial distinctions from the variation of the present 

 character, I think it will be conceded that, so far as any conclusions at 

 all are permissible, they confirm the inferences which have already been 

 drawn from the evidence of the more variable characters. It will 

 be noticed, moreover, that the North Sea and Plymouth data are 

 amalgamated, the percentages for Brest and Scilly approximate to the 

 percentages for the combined groups in a remarkably close manner, — 

 much more nearly than they do to the combined Irish percentages. 

 This result is in complete agreement with the results already described 

 for the characters previously discussed. 



XII. The Number of Dorsal Finlets. 



The number of dorsal finlets in my samples was never less than 4 

 nor more than 6 ; but there is a certain difficulty in enumerating them, 

 owing to the fact that the last ray of the second dorsal fin is sometimes 

 imperfectly separated off as an accessory, or incipient, finlet. As the 

 finlets and second dorsal fin are both modifications of a primitively 

 continuous fin (such as that which exists in Caranx trachurus, the 

 horse-mackerel, and its allies), it is desirable to include these incipient 

 finlets in an account of the variability of the number of finlets. They 

 have been included in the account of the second dorsal fin as a matter 

 of course, each incipient finlet counting as one dorsal ray ; but in the 

 present case, in order to reduce their value as compared with the fully 

 constituted and independent finlets, I have regarded them as half- 

 finlets. Thus, a fish which has 4 true finlets and one incipient finlet 

 has been recorded as having 4J finlets. 



The observed frequency of the different possibilities is recorded in 

 Table H (p. 295). 



The normal or modal number is, of course, 5. In the European 

 samples this number occurs in from 92 to 94% of the fish, but the 

 American race of mackerel is again distinguished from the European 

 samples by its greater variability in regard to this character, the modal 

 number being found in only 79% of the observed cases. In only 

 2% of the cases was the number reduced below 5, viz., 4^, but no 

 specimen was seen with only 4 finlets. On the other hand, 19% had 

 more than 5 finlets, viz., 12% with 5| finlets, and 7% with 6. 



The slight variability of the European fish in regard to this character 

 renders necessary the same treatment as was applied in the case of 

 the second dorsal fin, and I have therefore merged into a single 

 compartment the cases showing less than 5 typical finlets, and into 

 another compartment the cases with more than 5 typical finlets. 



