250 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
are susceptible of being grouped into six genera—Asta- 
coides (fig. 65), Astacopsis, Cheraps, Parastacus (fig. 64), 
Fic. 64.—Parastacus brasiliensis (4 nat. size). From southern Brazil. 
Engeus, and Paranephrops—on the same principle as 
‘that which has led to the grouping of the Northern forms 
into two genera. But the same convenience which has 
