Help with identification please

[thrubwell]

I found this "shell" on  a beach in northern Sardinia.

<a href="http://www.badongo.com/pic/6995896" target="_blank"><!-- m --><a class="postlink" href="http://www.badongo.com/pic/6995896">http://www.badongo.com/pic/6995896</a><!-- m --></a>

It's not like a normal shell. It's very light and brittle.

Thanks in advance for any help.

Al