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



Platl: XXI. 



\, 



Upper pair: Kidney-shell, Ptychohranchus phaseolus (Hildreth), from Obey River, Ky. (See p. 33.) 

 Lower pair: Rock pocketbook, Arcidens confragosus (Say), from IMississippi River. (See p. 34.) 



