digging a hole and found this shell

[jfornash]

4 inche in length , and I thought it was a bone at first, until I cleaned it off dous anyone know what it is?
<a href="http://www.badongo.com/pic/559037" target="_blank"><!-- m --><a class="postlink" href="http://www.badongo.com/pic/559037">http://www.badongo.com/pic/559037</a><!-- m --></a>