DISTINCTIVE CHARACTERS OF THE CRAYFISH. 



233 



spine. 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 



Fig. 61.— a, D, & G,Astaciis torrentinm ; B, E, & H, A. nohUj.i; C. F, & 

 I, A. nigrescen.1 (nat. size). A— C, Dorsal views of carapace ; D--F, 

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

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

 inclosing the areola. 



