Martin,
The main question is this: Do these molluscs turn yellow when you put them
in bleach??
 
Just kidding. I used to be hot on the freshwater clams back in the 1970s
(much to my ex-wife's chagrin). I spent many days each year in the
Tennessee, Alabama and Mississippi areas collecting. But the nicest
freshwater shells of all (in my opinion) came from a small creek east of
San Antonio, Texas (who would have guessed?). I still have two of the
smaller ones. The rest got sold after my divorce.
 
Best regards,
Jim Miller
Tallahassee, Florida