Tegulas! Black turbans, https://en.wikipedia.org/wiki/Tegula_funebralis

It was a big Tegula morning at Moonstone Beach at low tide this morning, with a cast of thousands piled up around the rocks. They used to be big part of the Chumash/Salinian seafood diet -- and vacant shells are very popular with hermit crabs. I'm pretty sure there's a ringer in here, at top center.

Done