Help me identify this shell from Sanibel Island

[novice]

I found this shell last week during a very low tide on a sand bar off the southern coast of Sanibel Island.  I found a Junonia the same day and wasn't sure what I had, so I took both shells into a shell shop for some help with idenification.  The proprietor looked through a couple of books and couldn't find it, so she thought it might be a land snail.  Any help?  Here's a couple of pictures:

<a href="http://www.badongo.com/pic/4833239" target="_blank"><!-- m --><a class="postlink" href="http://www.badongo.com/pic/4833239">http://www.badongo.com/pic/4833239</a><!-- m --></a>
<a href="http://www.badongo.com/pic/4833250" target="_blank"><!-- m --><a class="postlink" href="http://www.badongo.com/pic/4833250">http://www.badongo.com/pic/4833250</a><!-- m --></a>