Any shell button experts want to take a stab at this one?

This request is from a St. Louis area resident who is apparently working on
a shell button classification.

'As a member of the National Button Society, I have been asked to revise a
classification booklet of shell buttons. I have solicited buttons to
photograph for the booklet and received one this morning that I need help
identifying. A year ago, we took our daughter to the St. Louis Shell Show so
I immediately, today, looked up your website. My first guess on the button
is a cowrie but with the polished spots it is hard to tell. The shell is two
inches long and is set in a metal mounting with a button shank. I'm assuming
the bottom of the shell was sawed flat before mounting. Thank you so much. -
Shirley Case [log in to unmask]

Shirley's photo of the button is at
http://www.lwatrous.com/unknown/button.htm

Copy replies to Shirley's email address.

Thanks,
Larry Watrous
St. Louis, MO