232 Part III. — Eighteenth Annual Report 



Table XIII. 



Length of Skull in Percentage of Body-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 . 



— - 



162 



25-93 



25-68-26-18 



63 



25-48 



25-03-25-93 



83 



24-60 



24-15-25-05 





6 



49 



25-24 



24-69-25-79 



117 



24-62 



24-22-25-02 



79 



23 61 



23-21-24-01 



Aberdeen . . 



2 



64 



24-50 



24-05-24-99 









36 



24-03 



23-38-24-68 





6 



66 



24-00 



23-60-24-40 



51 



23-80 



23-40-24-20 



36 



23-44 



22-89-23-99 



Baltic . . . 



2 









30 



24-83 



23-98-25-68 









Again, it is seen that in this character also the Baltic specimens lie 

 betweeu the two other groups, but are somewhat nearer the southerly 

 North Sea group than that of Aberdeen. 



Skull-breadth. — The following Table, XIV., shows that in all the 

 groups, both immature and mature, a considerable difference exists 

 between the southerly North Sea specimens and those from Aberdeen 

 with regard to this dimension The average-difference over all the 

 groups amounts to l*84°/ 00 of the skull-length, the Aberdeen being 

 larger than those from the southerly North Sea. The fluctuations of the 

 averages, again, do not overlap in any of the groups, so that the proba- 

 bility amounts to a certainty in favour of this difference observed being 

 real. 



If this difference in skull-breadth be taken relative to the length of 

 body instead of the length of skull, it becomes — taking skull-length as 

 equal to one-fourth of the body-length — •46°/ 00 on the average. Hence, 

 independent of any difference in the skull-length, which has been shown 

 to be less in the northern than in the southern groups, we find that 

 there is an increase in skull-breadth in passing from south to north. For 

 a plaice of 300mm. this difference amounts to '9mm., and for one at 

 400mm. to 1-2 mm. 



We may briefly contrast the difference in skull-length with that in 

 skull-breadth. For plaice of 300mm. the Aberdeen group is less by 

 l*7mm. in the former character, but greater by *9mm. in the latter, than 

 the southerly North Sea group. As they stand, however, these two 

 differences are not comparable. The skull-length is more than twice the 

 skull-breadth, so that any change in the former is of the same relative 

 value as less than half that change in the latter. It would therefore 

 appear that the one difference is counterbalanced by the other, or that 

 what has been taken away from the Aberdeen group in skull-length is 

 exactly made up for by an increase in skull-breadth. 



A further reference to these correlated changes will be made in dis- 

 cussing the depth of skull. Meanwhile, it may be remarked that the 

 Baltic plaice group is again intermediate between the two other groups, 

 but that its average approaches more nearly to the southerly group than 

 to the northerly. 



