Hi Harry,

Thanks for your input.  Actually several people did contact me
privately.  The species you suggest have been mentioned more than once,
as has Pyganodon grandis.  I have sent some specimens to a couple of
knowledgeable people, so hopefully will have a definitive answer soon.
The shells are thin and toothless.  As for the "pond" they were found
in, it is hardly deserving of the name.  It's a tiny, presumably
manmade, water-filled depression about 50 to 60 feet in diameter, and
not more than 4 feet deep.  A little, shallow canal about 6 feet wide
passes under a small footbridge, to a similar pond on the other side.
When I was there last week, the water-filled area (due to drainage of
the pond) was not more than 25 to 30 feet wide.

Paul M.