Mangrove oyster. Found growing on prop roots.
This may be a one of the picture list that I saw on beach.
Saw several of these.
A few scant patches of oysters attached to the lower regions of the artificially-placed pile of rocks. (photos also posted on bowerbird.org.au)
Not really sure, but I donn't see what else it could be.
Chincoteague NWR, VA April 4, 2015
Shells on the beach