C. mcenas.—Right Dentary Margin.
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Carapace length in millimetres. Average excess of 1895 crabs over 1893 crabs. ! In thousandths of ( carapace length. In millimetres. 1 0 -1 1 1 -39 o -o i 11—12 2-09 0-02 1 2 -1 3 1 -87 0-02 13—14 1-56 0-02 14—15 1 -42 0-02
As these results seemed to indicate that a change in regard to these dimensions was taking place in the species, it was desirable to compare similar measurements in the adult. Fortunately Professor Weldon was able to supply me with 254 specimens of male C. mcenas with a carapace length ranging between 40 and 63 mm., taken at Plymouth at random in 1892—3 : and for comparison he procured 496 individuals collected at Plymouth in January of the present year and corresponding in size.
Measurements similar to those made on the young ones gave the following results :—In frontal breadth the 1892-3 crabs exceeded the 1896 crabs on an average by 8‘85 thousandths of their carapace length, which for an average length of 50 mm. is equivalent to 0'44 mm., while in the right dentary margin the 1896 crabs exceeded those of 1892-3 on an average by 3'1 thousandths, or an equivalent of O'lG mm., thus fully confirming the results arrived at in the young ones.
Whether these results indicate a permanent change in the species at Plymouth in respect to these particular dimensions of the carapace, tending to the establishment of a new variety, or whether it is a mere oscillation such as, for all we know, may be constantly going on in the relative dimensions of the various parts of the members of all species, can only be decided by further measurements, which, it is hoped, may be continued on the same species after another interval of two or three years. Meanwhile, the persistence^ with which the same tendency asserts itself in the twenty-six groups into which we have divided these crabs of 1892—3 and 1895-6 is remarkable, and may perhaps induce others to take measurements of other animals at definite intervals, and establish similar comparisons.
I wish to add my hearty thanks to Professor Weldon for suggesting the line of investigation and furnishing material and ever-ready help
