DISTINCTIVE CHARACTERS OF THE CRAYFISH. 233 



The elevation itself may be distinguished as the 

 post-orbital ridge. The flattened surface of this ridge is 

 marked by a longitudinal depression or groove. The 

 ABC 



FlG. 61. A. D, & Gr,A*tacvs torrentium ; B, E, & H, A. noUlis; C, F, & 

 I, .4. nirjrwrnx (nat. size). A C, Dorsal views of carapace ; D F, 

 side views of third abdominal somites ; G I, Dorsal views of telson. 

 a, b, post-orbital ridge and spines ; c, branchio- cardiac grooves 

 inclosing the areola. 



