Hi All,

I am trying to ID a rather large (120 mm) fresh water bivalve from the
southern USA.  I have visited some unio sites, but after randomly
downloading and discarding pictures for a half hour, I haven't found it.
 I don't expect a positive ID from a description, but if anyone has
suggestions, I can narrow down my search, and hopefully find it.  Or, if
necessary I can produce a picture and post it on the web.  But for now,
here is the description:

120 mm, rather solid and heavy, shiny black periostracum overlying a
pearly white exterior.  Sculpture consists of heavy concentric growth
ridges.  No radial sculpture or tubercles.  Umbos are near the anterior
end.  Anterior end is narrow and rounded, posterior end is wide, squared
off, and more inflated than the anterior end.  Interior is a lovely
violet-pink color. Anterior to the umbos are large hinge teeth on both
valves.  Posterior to the umbos, a sharp ridge or keel on the right
valve fits into a deep groove on the left valve. Any suggestions?

Regards,
Paul M.