Introductory.—I believe that, so far as concerns the more important species, sufficient information has been accumulated to allow of the deduction of a reliable conclusion.

Though it is certain that some local variation exists in the different parts of the North Sea, I have not found it possible to treat the area otherwise than as a single district. This is of the less importance since the variation seems to be very slight, and in any practical application of the results arrived at I do not see how it would be feasible to subdivide the district.