234 Part III. — Eighteenth Annual Report 



Table XV. 



Depth of Skull in Percentage of Skull-length. 



Region. 



Sex. 



Immature (250— 350nim. ) 



Mature (250-350mm.) 



Mature (350 — mm.) 



No. 



M. 



P.F. 



No. 



M. 



P.F. 



Xo. 



Iff. 



P.F. 



So. North Sea, 



2 

 6 



147 



47 



37-58 

 37-64 



37-13-38-03 

 36-99-38-29 



60 

 115 



37-10 

 37-88 



36 '35-37 "85 

 37-48-38-28 



80 

 77 



37-70 

 37-48 



37-05-38-35 

 37 03-37-93 



Aberdeen. . . 



2 

 <$ 



62 

 66 



39-13 

 39 -C3 



38-38-39-88 

 38-33-39-73 



50 



39-20 



38-60-39-80 



36 

 36 



39-00 

 38-69 



37-60-40-40 

 37-89-39-49 



Baltic, . . . 



2 



- 







30 



37-67 



36-82-38-52 









As in the case of the skull-breadth, therefore, the change in the skull- 

 length is counterbalanced by the change in skull-depth, and the slight 

 difference that occurs is in favour of a relatively greater increase in 

 breadth and depth than in length. 



As with the other dimensions which have been considered previously, 

 so with the skull-depth it is seen that the Baltic plaice are intermediate 

 between the southerly and northerly groups of the North Sea, but 

 approaches more nearly the southerly group. 



Skull. — We may now endeavour to compare the two Xorth Sea groups 

 with regard to the skull as a whole. It has been shown that as we pro- 

 ceed from north to south we meet with an increasing length of skull, but 

 a decreasing breadth and depth. The skull becomes more elongated, but 

 compressed posteriorly. It is, unfortunately, impossible to say exactly 

 what alteration in mass and volume this change in dimensions brings 

 about, but this should not be difficult to determine later. If we consider 

 the form of the skull, however, how by far the greater proportion of the 

 mass and volume is concentrated posteriorly, and how the elongated paras- 

 phenoid, vomer, and ethmoid anteriorly show but comparatively little 

 bulk, it seems reasonable to conclude that the equal " dimensional " varia- 

 tion from north to south is not accompanied by an equal variation in mass 

 and volume. It is more probable that the latter diminishes with the 

 diminution of the breadth and depth of the skull as we proceed from 

 north to south. 



If now, along with these results, we consider the changes in the skull 

 arising from growth-variability, we find that the two changes cannot be 

 confused with one another. It has been shown that the relative length 

 of skull decreases during growth, hence it might be thought that the 

 reason for the northerly plaice having a lesser skull-length than the 

 southerly w T as that they were older, or larger. Even apart from the fact 

 that the same average-sizes, and presumably, therefore, the same average- 

 ages have been compared throughout, this reasoning would at once be 

 refuted on turning to the changes in the breadth and depth of skull. 

 These dimensions, it has been shown, also decrease with age, and should 

 on this line of reasoning follow the change in skull-length and decrease as 

 we go from south to north. On the contrary, however, they increase. 



