DISTINCTIVE CHARACTERS OF THE CRAYFISH. 231 



males are commonly somewhat larger, and they almost 

 always have longer and stronger forceps than the 

 females. The general colour of the integument varies 

 from a light reddish-brown to a dark olive-green ; and 

 the hue of the tergal surface of the body and limbs is 

 always deeper than that of the sternal surface, which is 

 often light yellowish-green, with more or less red at the 

 extremities of the forceps. The greenish hue of the 

 sternal surface occasionally passes into yellow in the 

 thorax and into blue in the abdomen. 



The distance from the orbit to the posterior margin of 

 the carapace is nearly equal to that from the posterior 

 margin of the carapace to the base of the telson, when 

 the abdomen is fully extended, but this measurement of 

 the carapace is commonly greater than that of the abdo- 

 men in the males and less in the females. 



The general contour of the carapace (fig. 61), without 

 the rostrum, is that of an oval, truncated at the ends : 

 the anterior end being narrower than the posterior. Its 

 surface is evenly arched from side to side. The greatest 

 breadth of the carapace lies midway betw^een the cervical 

 groove and its posterior edge. Its greatest vertical depth 

 is on a level with the transverse portion of the cervical 

 groove. 



The length of the rostrum, measured from the orbit 



considerably smaller. Doubtless, the proportional rate of increment is 

 much the same, in the two kinds ; but in the English crajfish it haa 

 not been actually ascertained. 



