So there were two identical shells, but one had a hermit crab and the other was moving like a snail, with no visible crab legs. Both appeared to be dark unicorn shells.
Literally hundreds, maybe thousands of these things all over the beach. This was one of the larger ones, but there were ones smaller than an inch.
Little fellow somehow attached himself to my partner's hand while he was sifting through the algae. He was coaxed onto algae after being photographed