A List of the Crustacea of Wisconsin. 179 



The genus Oambarus is the only representative of the family 

 Astacida3 in the fresh waters of the United States east of the 

 Eocky Mountains. These animals, popularly known as " craw- 

 fish " or " crabs," are our largest crustaceans. The great number of 

 species, separated by characters generally obscure and difficult of 

 definition, many of them remarkably, inconstant, with an un- 

 doubted dimorphism of males and the not infrequent occurrence 

 of abnormal individuals, render the study of this genus particu- 

 larly perplexing. 



The following key will assist in separating the species herein 

 mentioned : 



A. — Rostrum toothed near apex, at least when young. 



B. — Eostrum long, pDinted; first abdominal legs of male 

 truncate ; third joint of third and fourth thoracic legs hooked. 



C, — Hands long and slender; fingers curved; cephalothorax 

 densely tuberculate. C. acutus. 



CC. — Hands short; cephalothorax smooth or nearly so. C. 

 stygius. 



BB. — Eostrum subquadrangular ; first abdominal legs of male 

 bifid ; hooks on third joint only, of thoracic legs. - 



D. — Tips of first abdominal legs of male nearly equal, straight 

 or slightly curved; two rows of teeth on lower border of brach- 

 ium very indistinct or absent. 



E. — Eostrum carinated above. C. ]jropinquus. 



EE. — Eostrum not carinated above ; hands large ; fingers gap- 

 ing at base. 0. placidus. 



DD. — Tips of first abdominal legs of male unequal and re- 

 curved. 



F. — Margins of rostrum converging in front ; first abdominal 

 legs of male long, thick, inner ramus swollen near apex. C. 

 wisconsinensis. 



A A. — Eostrum toothless. 



H. — First abdominal legs of male truncate ; three slender teeth 

 at apex. 0. gracilis. 



HH. — First abdominal legs of male ending in two short, thick, 

 recurved teeth. 



I. — Dorsal areola wide. C. hartonii. 



