DISTINCTIVE CHARACTERS OF THE CRAYFISH. 235 
edge of the carapace. Hach groove runs, at first, obliquely 
inwards, and then takes a straight course parallel with its 
fellow. The area thus defined is termed the areola; its 
breadth is equal to about one-third of the total transverse 
diameter of the carapace in this region. 
No such distinct lines indicate the lateral boundary of 
the region in front of the cervical groove which answers 
to the stomach. But the middle part of the carapace, 
or that which is comprised in the gastric and cardiac 
regions, has its surface sculptured in a different way 
from the branchiostegites and the lateral regions of the 
head. In the former, the surface is excavated by shal- 
low pits, separated by relatively broad flat-topped ridges ; 
but, in the latter, the ridges become more prominent, 
and take the form of tubercles, the apices of which are 
directed forwards. Minute sete spring from the depres- 
sions between these tubercles. 
The branchiostegite has a thickened rim, which is 
strongest below and behind (fig. 1). The free edge of 
this rim is fringed with close-set sete. 
The pleura of the second to the sixth abdominal 
somites are broadly lanceolate and obtusely pointed at 
their free ends (fig. 61, D); the anterior edge is longer 
and more convex than the posterior edge. In the females, 
the pleura are larger, and are directed more outwards and 
less downwards than in the males. The pleura of the 
second somite are much larger than the rest, and over- 
lap the very small pleura of the first somite (fig.1). The 
