I think I found it.  I found a list of common and scientific unio names
online.  Because of the interior color, I played a hunch and looked for
species called "purple ... something". or "pink ... somthing".  After
striking out on a few pinks and purples, I opened an image called
"purple pocketbook" (why don't marine shells have names like this?).
Anyway, the species that goes under that name is Potamilus purpuratus.
The image looks like my shell, but if there are other species similar to
this, I would like to know.  Thanks.

Paul M.