1890.] occurring in certain Decapod Crustacea. 449 



In measuring the length of the sixth tergum and of the telson, 

 these organs were removed from the body ; so that the small portion 

 of each which projects inwards into a fold of skin, and serves as an 

 attachment for muscles, is included in the total length. 



The individuals measured were all adult females ; they were col- 

 lected from widely separate places. The first sample measured 

 consisted of 400 individuals from Plymouth Sound ; a second sample, 

 containing 300 females, was obtained from Southport by Mr. W. 

 Garstang ; and a third sample, of which 300 were measured, was 

 sent to me from Sheerness by Mr. W. H. Shrubsole. 



Total Length of the Carapace. — The curve obtained by treating the 

 total length of the carapace of the Plymouth specimens in the way 

 ■described is shown in the woodcut, Fig. 2. 4 



Fia. 2. 



It will be seen that the median ordinate has a length of 250'52, 

 which is therefore the value of M. The ordinate at 25° is 255'07 = 

 M + 4-55 ; that at 75° is 246-00 = M— 4*52; so that the mean value 

 Q = 1(4-55 + 4-52) = 4-53. 



