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 between 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 Fnglish crayfish it has 
not been actually ascertained, 
