Johan,
  I collect mostly fossils but they may be a species of Chione.  They are
in the Veneridae family along with Mercenaria.  They are smaller and tend
to have concentric "frills".  I cannot help you with color as all of my
Chione are fossilized and are various shades of white and grey.  Check out
Abbott's American Seashells and you will probably find illustrations of
all the common Chione from Florida.
 
Charlie
******************************************************************************
Charlie Sturm, Jr
Research Associate - Section of Invertebrate Zoology
                     Carnegie Museum of Natural History
Assistant Professor - Family Medicine
 
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On Mon, 6 Jul 1998, J.A. Dekker wrote:
 
> Dear all,
>
> On vacation on Sanibel i found some bivalves, looking like Mercenaria,
> cream or greyish white with brown stripes or bloths on it and fine ribs on
> the exterior.
> The interior is white, they are about 2 inch big. Any idea, which species
> this will be?
>
> Johan Dekker.
>