Bull. U. S. 13. F., 1917-18. 



Plati^ XX. 



Top: Bullhead, Plcurobcvia acsopus (Green), from Wabash River. (See p. ss-) 



Middle pair: Dromedary mussel, Dromus dramas (Lea), from Cumberland River. (See p. 2,S-) 



Lower pair: Fan-shell, Cyproge^iia irrorata (Lea), from Cvunberland River. (vSee p. i,:^.) 



