DISTINCTIVE CHARACTERS OF THE CRAYFISH. 283 
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 
Fic. 61.—A, D, & G, Astacus torventium ; B, E, & H, A. nobilis; C, F, & 
I, A. nigrescens (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; v, branchio-cardiac grooves 
inclosing the areola. 
