Can You Help Me Identify This Shell?

[verla]

This shell measures 5-5/8 in. by 3-1/8 in. Color is mostly black with a white frosted appearance.  Origin unknown.
Picture: <a href="http://www.badongo.com/pic/938939" target="_blank"><!-- m --><a class="postlink" href="http://www.badongo.com/pic/938939">http://www.badongo.com/pic/938939</a><!-- m --></a>
Thanks