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



The only case in which an undoubtedly asymmetrical result was 

 obtained is that of the frontal breadth of the Naples specimens. 

 From an inspection of the curve of distribution of these magnitudes, 

 I was led to hope that the result obtained might arise from the 

 presence, in the sample measured, of two races of individuals, 

 clustered symmetrically about separate mean magnitudes. Professor 

 Karl Pearson has been kind enough to test this supposition for me : 

 he finds that the observed distribution corresponds fairly well with 

 that resulting from the grouping of two series of individuals, one 

 with a mean frontal breadth of 630' 62 thousandths, and a probable 

 error of 12'06 thousandths ; the other with a mean breadth of 

 654'66 thousandths, and a probable error of 8*41 thousandths. Of the 

 first race, Professor Pearson's calculation gives 414'5 individuals, of 

 the second, 585'5. The degree of accuracy with which this hypo- 

 thesis fits the observations may be gathered from fig. 3. 



FIG. 3. Diagram to show the distribution of all observed frontal breadths in 

 the Naples specimens. The horizontal scale represents thousandths of the cara- 

 pace length, the vertical scale numbers of individuals. Each ordinate of the upper 

 dotted curve is the sum of the corresponding ordinates of the two component 



We may, therefore, assume that the female Carcinus mamas is 

 slightly dimorphic in Naples with respect to its frontal breadth ; and 

 that the individuals belonging to the two types are distributed in the 

 proportion of nearly two to three. 



