Dan,
It is probably the European exotic zebra mussel Driessina polymorpha (Pallas,
1771).  There is a good picture of the individual shell in Kevin Cummings book on
Midwest Mussels, but that individual shell picture insert in the book did not get
into the on line version of a clump of Dreissina on a unio:
http://www.inhs.uiuc.edu/cbd/musselmanual/page176_7.html
The Dreissina is listed in that online guide under the Lampsilinae, which of course
it is not.  The small deltoid cup shaped valves are common on the beaches of larger
rivers/ lakes in the upper midwest, alas. The beach specimens do not always have the
strong banding that gives the species its common name.

Alan Gettleman
Merritt Island, FL

Dan Yoshimoto wrote:

> Dear All Mytilid Fans,
>         One of the members the other night at our Lost Coast Shell Club
> meeting, brought a fresh water mytilid to identify.  As Neither I nor the
> others in the group are "Fresh Water Bivalve Fans" I was asked to post the
> following on Conch-L.  The data tag reads as follows.
>
> MYTILIDAE
> No Genus/species
> Collected at Chesterton,
> Lake Co., Indiana
> On Southern tip of Lake Michigan.
> Dead on beach.  Loose valves
> Leg: Lisa Ross  4/13/2001
>
> If you think that you can identify the small 23 mm mytilid, let me know and
> I'll send a specimen or two for you to check out.  If you live in the area
> you might already know what species this is.
>         Now that you all returned from COA Convention, it's time we all go
> back to "play".
> Dan Yoshimoto
>
> Dan & Hiromi Yoshimoto
> 1164 Vista Dr.
> Eureka, California
> 95503-6018
> U.S.A.
> e-mail: [log in to unmask]
>
> ----------------------------------------------------------------------------
> --------------------------------------------------------------------------------