Thanks Andy!

Wow!  What a wealth of information!  I think part of your post may answer my
question re: Personidae/Distorsiidae.  I'll have to reread it more carefully
to be sure.

Any thoughts on where Potamididae came from?  As far as I know there is not,
nor has there ever been an extant genus Potamida (or Potamidus/Potamidum).
Could it be this family was named after a fossil genus?

Regards,
Paul M.

Response to Kurt's last post, to avoid an extra posting by me:
Why break them?  Couldn't resorption be determined by x-raying the shells?