I am trying to identify this shell

[artist1]

I don't have a photo of the shell itself but it is the largest shell in this painting:

<a href="http://artforlife-llc.com/3%20shells%20alt.jpg" target="_blank"><!-- m --><a class="postlink" href="http://artforlife-llc.com/3%20shells%20alt.jpg">http://artforlife-llc.com/3%20shells%20alt.jpg</a><!-- m --></a>

The shell is approximately 11" long, the color in the painting is correct, and the person who gave it to me said that she found it on the beach in South Carolina after a hurricane. I'd like to be able to identify the shell correctly on a new drawing that I'm working on.