Bull. U. S. B. F., 1919-20. 



Plate VII. 



M7^ 





I-'iG. I. — Black River, Ark., a verj'' productive mussel stream. 



Fig. j. — Red River. ucarCampli, La., a turbid stream with caviu;; banks and shifting; bottom, quite unfavorable 



for mussels. 



