1893.] 



Correlated Variations in Carcimis moeiias. 



319 



deviations from the average size of every organ measured are grouped 

 symmetrically about the average, and occur wi~h a frequency corre- 

 sponding closely to that indicated by the probability integral ; and 

 (2) that the " degree of correlation " between a given pair of organs 

 is approximately the same in each of five local races of the species 

 (' Roy. Soc. Proc.,' vol. 47, p. 445, and vol. 51, p. 2). In what follows 

 I shall describe the results obtained by measuring certain parts of 

 the shore crab (Carcinus moenas) in two samples, one from the Bay 

 of Naples, and one from Plymouth Sound, each sample consisting 

 of 1,000 adult females. 



1. The Variation of Individual Organs. 



The measurements made were as follows : 



1. The total length of the carapace (fig. 1, AB), in a straight line from 

 the tip of the median inter-orbital tooth to the middle of the posterior 

 margin. 



FIG. 1. Diagram to show the parts of the carapace measured. The diagram 

 is drawn to scale, the right half representing a perfectly average Plymouth crab, 

 the left an average crab from Naples. 



2. The total breadth of the carapace, in a straight line from tip to 

 tip of the posterior lateral teeth (fig. 1, EF). 



