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 setz on this region in the female; while, in 
the male, the sete 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 sete. 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 sete (fig. 35, 
p. 186). 
The importance of this long enumeration of minute 
details* will appear by and by. Itis 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 
training of the faculty of observation. 
