312 



Part III. — Ev/hteenth Annual Report 



Fix-rays and Finlets. 



The variations in the numbers of fin-rays and finlets are given in 

 Table IXa. (p. 319). The average number of fin-rays in the first and second 

 dorsal and anal fius was calculated for males and females separately 

 (Table V.), and for the two sexes combined (Table Va). The same 

 was done for the dorsal and anal finlets, and the results are given 

 with the former. Owing to the fact that the majority of the mackerel 

 from each locality are of nearly one size, it is impossible to satisfactorily 

 break them up into groups by which any change in the number of fin- 

 rays due to growth might be proved to be present or not. So far as 

 sexual differences are concerned, Table V. does not give any proof 

 of their existence. The only difference between the averages of the males 

 and females is to be found in the second place of decimals. This holds 

 good for each fin, and for the finlets in the mackerel from the three 

 localities. Eleven of the Stornoway specimens, of undetermined sex, 

 give a slightly different average for the rays of the first dorsal, but in so 

 variable a character eleven specimens are too few upon which to place 

 any importance. If, now, the males and females are taken together, and 

 a comparison made between the Means for the three localities, a little 

 divergence is seen in the case of the first dorsal fin. In respect to this 

 character the Means for iVberdeen, the Clyde, and Barra and Stornoway 

 are respectively 135, 13 '4, and 13 '76, In place of comparing the 

 observed averages, it is necessary to compare the fluctuations of the 

 Means (Table Va.). In the case of the first dorsal the fluctuation is 

 13-27-13-73 for Aberdeen, that for the Clyde 13-14-13-66, and that for 

 Barra and Stornoway 13-41-14-11. In order that the Means may be 

 regarded as differing from one another, their fluctuations must not overlap. 

 But in this case all three do so, so that the probability is that as regards this 

 character there is no racial distinction between the mackerel of the three 

 districts. On reference to the Table it is seen that the fluctuations of the 

 Means overlap in the case of the rays of the second dorsal and anal fins 

 and the finlets. So far, then, as these characters are concerned, no racial 

 differences exist between the three groups. 



Vertebrae. 



The number of vertebrae was found to be very constant, viz., 31. 

 One mackerel alone, 31 -3cm. in length, from the Clyde, had a greater 

 number, viz., 32. Xo example was found with less than 31 vertebrae. 



The Question of Sexual Differences. 



So far as this research goes, none of the characters examined can be 

 claimed to have any sexual variability. On reference to Table III. it 

 may be seen that the relation between the male and female averages is 

 comparatively irregular. While in one group of mackerel (Aberdeen) the 

 male average for the distance of the origin of the pectoral fin from the 

 snout (d Pf.) is greater than the female average, in the Clyde the reverse 

 is the case, and in the Barra fishes the male average is again the greater. 

 In three characters only — the distance of the first dorsal (d 1 D), 

 the distance of the anus (d A), and the distance of the anal spine (d As) 

 from the snout respectively — is the female average greater than that of 

 the male in the mackerel from all three localities. The male average, on 

 the other hand, exceeds the female in three characters — viz., the length 

 of the skull (I Sk), the length of the mandible (I M) 3 and the length of 



