of the Fishery Board for Scotland. 233 

 Table XIV. 

 Breadth of Skull in Percentage of Skull-length. 



Region. 



Sex. 



Immature (250 350mm.) 



Mature (250-350mm.) 



Mature (350— mm.) 



No. 



M. 



P.F. 



No. 



M. 



P.F. 



No. 



M. 



P.F. 



So. North Sea, . 



? 



154 



47*49 



47-19-47-79 



63 



47-05 



46-40-47-70 



82 



46-32 



45-72-46-92 





df 



49 



47-41 



46-76-48-06 



106 



47-00 



46-45-47-55 



73 



46-58 



45-88-47-28 



Aberdeen, . . 



$ 



62 



49-05 



48-30-49-80 









36 



48-14 



47-34-48-94 





6 



66 



49-41 



48-71-50-11 



51 



49-00 



48-00-50-00 



36 



48-40 



47-70-49-10 



Baltic, . . . 



2 









30 



47-80 



46-65-48-95 









Skull-depth. — As in the case of the skull-breadth, the sex- and growth- 

 variability of this dimension does not affect the conclusions that may be 

 drawn from the Tables as they are arranged. The same sizes and the 

 same sexes may be compared directly. 



It is seen that in all the groups the averages for the southerly North 

 Sea are much smaller than those for Aberdeen. The difference between 

 the various groups is very much the same in all, varying only from 1*21 

 to 1*55, and the average amounts to l*35°/ 00 of the head-length. The 

 amount of probability for the actual existence of these differences is very 

 great, being 1000 to 1 in the first three groups, and but little less than 

 that in the last two in spite of the large fluctuations of the averages in 

 the groups of mature specimens from Aberdeen. These large fluctuations, 

 as already pointed out, come from the relatively small number of specimens 

 in those groups. 



When this difference in the skull-depth relative to the skull-length is 

 taken relative to the body-length it becomes *34. There is thus an 

 increase in the skull-depth of -34 0 / O0 of the body-length as we pass from 

 south to north. For a plaice of 300mm. this difference means an increase 

 of '68mm., and for one at 400mm. an increase of '88mm. 



We may again compare this increase of skull-depth with decrease of 

 skull-length in the northern plaice. For a plaice of 300mm. the northern 

 group is greater by -68mm. in skull-depth, but less by l-7mm. in skull- 

 length than the southern group. But as they stand these two proportions 

 are not directly comparable. The skull-depth is on an average about 

 38°/ 00 of the skull-length, or a little more than a third, and hence, if a 

 change of l-7mm. occurs in the skull-length, the proportionate change in 

 the skull-depth should only be '64mm. 



[Table. 



