DISTINCTIVE CHAKACTERS OF THE CRAYFISH. 235 



edge of the carapace. Eacli groove rims, 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 sculptm'ed 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 

 dii'ected forwards. Minute setae 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 setae. 



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 ver}^ small pleura of the first somite (fig. 1). The 



