Help identifying a shell for a total novice

[tom]

Here are two pictures of a shell I found in 2007 while snorkeling in Roatan.  (The pictures are fairly large - sorry about that.)  I am just curious what it is since it is very pretty and I have have never seen one with both halves attached and no holes in the shell itself.  My wife has it in our curio cabinet because it is that nice. I am total novice at this, so any help would be greatly appreciated.  You can just email directly at <!-- e --><a href="mailto:tpage@msu.edu">tpage@msu.edu</a><!-- e -->.  Thanks in advance. <a href="http://www.msu.edu/user/tpage/shell2.html" target="_blank"><!-- m --><a class="postlink" href="http://www.msu.edu/user/tpage/shell2.html">http://www.msu.edu/user/tpage/shell2.html</a><!-- m --></a>