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 setae on this region in the female ; while, in 

 the male, the setae 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 

 posterior 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 b}'. 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 zoology will find the comparison of a 

 lobster with a crayfish in all the points mentioned to be an excellent 

 trainings of the faculty of observation. 



