322 



Prof. W. F. R. Weldon. On certain [Nov. 16, 



spite of some considerable discrepancies, the general agreement 

 between the second and third columns of the table is undeniable. 



Table I. Distribution of Lengths of Antero-lateral Margin of Cara- 

 pace in 999 Female Carcinus moenas from Naples. 



The right antero-lateral margin of the Plymouth individuals, when 

 treated in a similar way, gave the following results : 



Arithmetic mean 762 '70 thousandths. 



Mean error 977 



Modulus 17-29 



The frequency with which individual deviations from the average 

 occur is compared with that indicated by a probability equation of 

 the appropriate modulus in Table II. 



These two examples will give a fair idea of the extent to which 

 the distribution of the observed magnitudes of each organ about the 

 mean of all of them corresponds to that indicated by the probability 

 equation. A similar treatment of every other set of measures would 

 serve no useful purpose ; it will be sufficient to give, in the following 

 table, the mean value, and the probable error of distribution about 

 that value, of every organ measured. The probable error is given 

 below, instead of the mean error, because it is the constant which has 



