Mystery Shell Found on Sanibel Island, FL

[kiki]

After being an avid collector of Sanibel Island treasuers for 17 years, I have found a shell that I can not identify.  I found it in January 2009 after a week of strong Northwest winds.  Can anyone offer a suggestion as to what kind of shell this is?  

I've added a picture, but you should the shell ius rather light weight and ribbed in the body.

<img src="http://i68.photobucket.com/albums/i24/rebekah_rain/DSCN0227.jpg" alt="http://i68.photobucket.com/albums/i24/rebekah_rain/DSCN0227.jpg" style="border:0" />
<img src="http://i68.photobucket.com/albums/i24/rebekah_rain/DSCN0224.jpg" alt="http://i68.photobucket.com/albums/i24/rebekah_rain/DSCN0224.jpg" style="border:0" />