Shell ID help?!

[lin2i]

Hello!  This is my first post here, but hopefully you'll be able to help me!

I'm an archaeologist and I am trying to find out what kind of shell this (below) is.  Although it was found in Sheffield, England, it is likely to have come from far away and be imported for button making (the cut outs are from button making) probably in the 19th century.

It would be really interesting and useful if anyone has an idea (even just a common, general name) for shells like this.

Many thanks,

Linzi

<img src="http://img86.imageshack.us/img86/6420/img1544wi7.jpg" alt="http://img86.imageshack.us/img86/6420/img1544wi7.jpg" style="border:0" />
Shot with <a href="http://profile.imageshack.us/camerabuy.php?model=Canon+PowerShot+A640&make=Canon" target="_blank">Canon PowerShot A640</a> at 2008-03-07