lis muscular impressions obvious: j9«//6«/ impression some- 

 what truncate before, and not extending anteriorly beyond 

 the line of the muscular impression. 



Var. a. Within, except on the anterior margin, purple 

 or livid. 



OBSERVATIONS. 



It occurs in the Wabash and Ohio rivers. In order to 

 avoid confusion I may mention that the present species is 

 in many of the European collections, sent by me within 

 the last five years, under the name of Flagellatiis. 



REFERENCE TO THE PLATE. 



The lower figures, are the outer and inner views of the shell. 



UNIO RIDIBUNDUS. 



SPECIFIC CHARACTER. 



Subquadrate ; emarginate before and denticulated on 

 the anterior basal angle. 



DESCRIPTION. 



Shell transverse, subquadrate oval, olivaceous, radiate 

 with capillary deep green lines ; summits somewhat pro- 

 minent, nearly terminal : cavity of the hinge membranes 

 oval, rather larger behind the beaks than broad : posterior 



PLATE v. 



