28 



expressing the breadth of the carapace and tail segments in 

 proportion, thus : — 



Thus in a crawfish whose carapace measures 100 mm. (from 

 the tip of the rostrum to the posterior border of the carapace) 

 the carapace is 3 mm. narrower in the female, and the tail 

 15 mm. broader in the first and third segments, 13 in the 

 second, 14 in the fifth, 11 in the fifth, and 7 in the sixth. A 

 female and a male of equal length of carapace are therefore 

 readily distinguished as there is a difference of 15 mm. in 

 the greatest breadth of the tail. This is enough to be readily 

 detected on a mere glance at the fish. 



(5) Though the carapace is narrower as measured by its 

 greatest breadth, the under side or sternum between the last 

 pair of walking legs is broader in the female than in the male. 

 This difference is well marked and is quite evident when 

 pointed out. Thus, in the third female in the tables (115 mm. 

 in length) the breadth of the sternum as measured by the 

 distances between the points of articulation of the five pairs 

 of walking legs were, as compared with those of the third male, 

 (measuring 122 mm. in length) as follows : — 



or taking the length of carapace as 100 : 



