204 THE FRESH-WATER MUSSEL [CH. VI 



ORDER FILIBRANCHIA. 



Family MYTILID^E. Byssus well developed : anterior adductor 

 muscle small. 



genus Dreissensia (van Ben), shell equivalve, inequilateral, swollen, 

 umbones anterior terminal; below umbo a triangular shelf 

 for support of anterior adductor muscle ; ligament internal ; 

 hinge toothless or with minute cardinals ; anterior adductor 

 muscle impression small, posterior large. Siphons pro- 

 minent. 



Dreissensia (Dreissena) polymorpha (Pall.), l", shell sea-mussel- 

 shaped and keeled in the middle of both valves. 



