324 Pari III.- — Eighteenth Annual Report 



Table XII. 





A. 







B. 







C. 





Variations in Dorsal Com- 

 bination. 



Variations in Anal 

 nation. 



Combi- 



Additional Variations in 

 Dorsal and Anal Com- 

 binations. 



2nd Dorsal 

 Fin-rays 



and 

 Finlets. 



Anal 

 Fin- rays 



and 

 Finlets. 



Fre- 

 quency. 



Anal 

 Fin-rays 



and 

 Finlets. 



Dorsal 

 Fin -rays 



and 

 Finlets. 



Fre- 



quency. 



Dorsal 

 Fin-rays 



and 

 Finlets. 



Anal 

 Fin-rays 



and 

 Finlets. 



Fre- 

 quency. 









16 



17 



38 



16 



14 



1 



14 



17 



1 



18 



17 



7 



16 



16 



8 



16 



17 



41 









16 



18 



3 



18 



17 



13 



17 



17 



351 



18 



16 



1 



19 



17 



1 









18 



18 



3 



Correlation behveen the Dorsal Couple ( 2nd Dorsal Fin-rays and Dorsal 

 Finlets) and the Anal Couple (Anal Fin-rays and Anal Finlets). 



For the purpose of this comparison, each set of fin-rays and finlets is 

 added together. The normal number is 17 in each case. The correlation 

 which is found to exist is as follows : — That a variation from the normal 

 number in the dorsal couple is accompanied by the normal number in 

 the anal couple, and vice versa. This proposition will be modified later, 

 when the exceptions to this rule are considered. In Tables X. and XI. 

 are set forth the various combinations of fin-rays and finlets which were 

 present in 468 mackerel (£ and $) ; and in Table XII. the number 

 of fin-rays and finlets are added for each couple. In the divisions A and 

 B all the variations which agree with the above generalisation are 

 entered. In division C are the exceptions. They are 16 in number. 

 One variation, which occurred eight times, is a decrease in the dorsal 

 couple, accompanied by a similar decrease in the anal — viz., 16 in each; 

 while a second variation, in which both couples were equally increased — 

 viz., 18 in each — occurred three times. In these two cases, then, we have 

 accompanying a decrease or an increase in one couple an equal decrease or 

 increase in the other. There remain five cases, in four of which a decrease 

 in one couple was accompanied by an increase to the same amount in the 

 other couple — viz., 16 and 18 and 18 and 16. The last exception — viz., 

 dorsal 16, anal 14 — is one in which an abnormally low number of anal 

 fin-rays — viz., nine — was found. This reduction is possibly due to injury. 

 Two other cases in which abnormally low numbers were found — e.g., in 

 one, two dorsal finlets and in the other eight dorsal fin-rays — have been 

 left out, since they were evidently due to injury. A generalisation which 

 would include all the cases (with the exception of the three supposed 

 abnormal cases) is as follows : — A variation in one couple is accompanied 

 by the normal number, or by an equal deviation in the same or opposite 

 sense in the other. It is very possible and probable that the variations 

 in the dorsal and anal couples may be sometimes correlated with varia- 

 tions in the number of caudal rays, and such a relation may be present in 

 the case of those exceptions which required a widening of the proposition, 

 I have, however, no data bearing on the number of caudal rays. 



