242 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



Comparing a male and a female of the same size, the 

 triangular area between the bases of the penultimate and 

 ante-penultimate thoracic limbs is considerably broader 

 at the base in the female. In both sexes, the hinder 

 part of the penultimate sternum is a rounded transverse 

 ridge separated by a groove from the anterior part ; but 

 this ridge is much larger and more prominent in the 

 female than in the male, and it is often obscurely divided 

 into two lobes by a median depression. Moreover, there 

 are but few setse on this region in the female ; while, in 

 the male, the setse are long and numerous. 



The sternum of the last thoracic somite of the female 

 is divided by a transverse groove into two parts, of which 

 the posterior, viewed from the sternal aspect, has the 

 form of a transverse elongated ridge, which narrows to 

 each end, is moderately convex in the middle, and is 

 almost free from setae. In the male, the corresponding 

 X^osterior division of the last thoracic sternum is produced 

 downwards and forwards into a rounded eminence which 

 gives attachment to a sort of brush of long setae (fig. 35, 

 p. 136). 



The importance of this long enumeration of minute 

 details * will appear by and by. It is simply a statement of 

 the more obvious external characters in which all the 

 full-grown English crayfishes which have come under my 



* The student of systematic zoolog-y will find the comparison of a 

 lobster with a crayfish in all the points mentioned to be an e::ccllent 

 training- of the faculty of observation. 



