322 



Part III. — Eighteenth Annual Report 



Dorsal Finlets. 





Second Dorsal 



Fin-rays. 





11 



12 



13 



14 



4 





5 





4 





5 



52 



392 





16 



1 



6 



1 



3 









It is thus evident that the normal or modal number of fin-rays of the 

 second dorsal (12) is associated with the two variations (4 and 6) of the 

 finlets, and in the present instance that association occurs more frequently 

 than what might be called the compensating association of a higher 

 number of fin-rays, e.g., 13, with a lower number of finlets, e.g. 4. 

 The following Table gives an analysis of 476 Scottish mackerel in respect 

 to the variation in the number of anal finlets and their association with 

 variations in the number of the anal fin-rays : — 









Anal 



Fin-rays. 







Anal 

















Finlets. 



















9 



10 



11 





12 



13 



14 



4 











2 



6 



1 



5 



1 



2 



44 





396 



12 





6 





1 



6 





3 







It is here seen that in this case also a similar relation is found, viz., 

 that the normal or modal number of fin-rays (12) is associated with both 

 variations from the modal number of finlets. The following generalisa- 

 tion is then warranted, viz., that the modal number of rays of the second 

 dorsal and anal fins (12) is to be found associated with the two varia- 

 tions (4 and 6) of the dorsal and anal finlets ; and, further, that with a 

 variation in the number of finlets either above or below 5, if the modal 

 number of the rays of the related fin is not found associated therewith, 

 the rays will be found to vary in number from 1 2 inversely as the finlets 

 vary from 5. 



It is, again, apparent from these two Tables that, as Garstang records in 

 his fourth generalisation, the normal number of finlets (5) is associated 

 with a wide range of variation in the number of fin-rays. A converse of 

 the generalisation made above with regard to the variations in the finlets 

 holds good also for variations in the fin -rays, viz., that the modal num- 

 ber of finlets is found associated with most of the variations in the 

 number of rays ; and, further, that with a variation in the number of 

 finlets either above or below 12, if the modal number of finlets be not 

 associated therewith, the finlets will be found to vary in number from 5 

 inversely as fin-rays vary from 12. This applies to both dorsal and anal 

 couples (fin-rays and finlets). These two propositions may be stated more 

 briefly : — 



