of the Fishery Board for Scotland. 231 

 Table XII. 



Length of Tail in Percentage of Body-length. 



Region. 



Sex. 



1 mmature (250 -350mm . ) 



Mature (250-350mm.) 



Mature (350— mm.) 



No. 



M. 



P.F. 



No. 



M. 



P.F. 



No. 



M. 



P.F. 



do. jn ortu feea . 



V 



162 



30-51 



30-06-30-96 



63 



29-93 



29-18-30-68 



84 



29-38 



28 73-30-03 





6 



49 



29-94 



28-99-30-39 



117 



30-02 



29-51-30-53 



78 



28-94 



28-29-29-59 



Aberdeen . . 



2 



64 



29-53 



28-93-30-13 









36 



29-89 



29-04-30-74 





6 



66 



29-21 



28-36-30-06 



51 



29-77 



28-92-30-62 



36 



29-75 



28-60-30-70 



Baltic .... 



2 









30 



31-00 



29-95-32-05 









Average for North Sea, 29 '77. 



Skull-length. — It has been shown that this character possesses both 

 sex- and growth-variability, but more especially the latter. It is there- 

 fore necessary to compare groups of the same sex and size, and this can 

 readily be done from the Table adjoining. 



Considering first the immature specimens, the Table shows that both 

 for males and females there is a considerable difference between the 

 southerly and the northerly North Sea. The latter have a shorter skull- 

 length than the former. The average difference is l , -335°/ o0 of the 

 body-length. The fluctuations of the averages, further, do not overlap in 

 either case, so that the probability of the observed difference corre- 

 sponding to a real difference in nature is at least 1000 to 1. 



A comparison of the male mature specimens between 250 and 350mm. 

 leads to the same conclusion. The difference between the two groups is, 

 however, not so great, being *82, and the fluctuations of the averages, 

 although they do not overlap, are not so distinct from one another as in 

 the former cases. Yet, from the fact of the fluctuations not overlapping, 

 the probability of the observed difference being real is almost as great as 

 in the above cases. There seems to be a further reduction in the differ- 

 ence between the two groups in the mature specimens above 350mm. 

 The averages of the Aberdeen specimens are certainly smaller than those 

 of the southerly North Sea for both sexes, but the average-difference is 

 only -37, and the fluctuations of the averages overlap in both cases. In 

 the case of the females, the probability that the observed difference is 

 real is 19 to 1, but in the case of the males it is only 1 to 1. As partial, 

 if not the whole, cause of the overlapping of the fluctuations may be 

 considered the comparatively small number of specimens in the Aberdeen 

 groups. 



Regarding the groups as a whole, the southerly North Sea specimens 

 have a longer skull-length than those from Aberdeen, the average-differ- 

 ence for all five groups being -846, and the probability is greatly in 

 favour of this difference being real. For a plaice of 300mm. this means 

 a difference of 1 -7mm. between the two groups, and for one of 400mm. a 

 difference of 2-2mm. 



