every 1 mm. of growth in carapace length, or twenty-five groups' for the whole range of 10—15 mm. carapace length. The numbers, contained in the separate groups ranged from seventy-two in the smallest group to 178 in the largest group. The arithmetical mean and mean error in each group is set out in Table I infra.
Similar measurements were made in the case of 1,957 young male C. mcenas from Plymouth of the year 1895. These were likewise divided into groups differing by 0‘2 mm. of carapace length : and the numbers contained in the twenty-five groups between 10 and 15 mm. carapace length ranged from thirty-four in the smallest one to 111 in the largest. The arithmetical means and mean errors are given in Table I infra.
On comparing the two sets of measurements (expressed in termsof the carapace length which was taken as the unit) it appears, asregards the “ frontal breadth,” that in every one of the twenty-five groups without exception the average size of the frontal breadth in the 1893 crabs exceeded that of the 1895 crabs of corresponding size. Seeing how small the groups are the result is a striking one, and is. given in greater detail in the following Table :—
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C. mcenas.—Frontal Breadth. Carapace length in millimetres. Average excess of 1893 crabs over 1895 crabs. In thousandths of carapace length. In millimetres.
10—11 6-30 0-07 11—12 7-29 0 '08 12—13 6'73 0-08 13—14 5-26 0*07 14—15 3-53 0 05
On the other hand, if the species in 1895 has a smaller average frontal breadth, it compensates for the deficiency by having a larger right dentary margin. This was found to be the case in twenty-three out of the twenty-five groups, the two non-conformist groups lying one near each end of the range. The arithmetical means and mean errors are given in Table I infra,and the results, tabulated in a corresponding form to those of the frontal breadth measurements, are as follows : —
