help me to find out about these big and old seashells

[enel]

I got hold on to these very old shells that were buried in the ground in a pyramid (3-5 feet deep) on Coronado Island in California. Probably a grave of a small animal - it was overgrown with roots but somewhat hollow under the "pyramid" of shells. It's not known how long ago they were buried - there's a house next to the excavation area built in 1932 and the area has never been touched except for gardening since the house was built.
I could not find anything like these online. Could you please let me know what they are and possibly what they might be worth if I'd sell them.
You can see the image (not professional, sorry <img src="images/smiley/smile.gif" alt="" border="0" />) at <a href="http://www.flickr.com/photos/49164227@N05/?saved=1" target="_blank"><!-- m --><a class="postlink" href="http://www.flickr.com/photos/49164227@N05/?saved=1">http://www.flickr.com/photos/49164227@N05/?saved=1</a><!-- m --></a>
Feel free to ask more questions.
Thank you <img src="images/smiley/smile.gif" alt="" border="0" />