Most of these oysters are encrusted with worms, barnacles, and boring shells.
Here, you can see a Petricola lithophaga in the upper valve.

This old shell was on a layer of rollers, between large blocks covered of its congeneric.
I often noticed that these shells are thicker and rougher when living on this kind of bottom
than those who live directly attached in crevices of the limestone bank,
the most foliated being those attached to rocks and boulders.