These are found in the salt marsh and the bay mudflats.  Very easy to spot because there are literally hundreds of these California Horn Shells.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/33466410@N00">Jerry Kirkhart</a> from Los Osos, Calif.
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jkirkhart35/2132219023/">Ceredithidea californica</a>