Dimensions of Parts of the Carapace o/Carcinus moenas. 197 

 C. moenas. Right Dentary Margin. 





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 G. mo&nas 

 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 0'16 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 al) 

 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 suggest- 

 ing the line of investigation and furnishing material and ever-ready 

 help. 



