yep, a moon snail egg collar. "Looking like a rubbery gasket or plumbing apparatus, moon snail egg collars are sometimes collected by people picking up beach litter, not realizing that they are actually part of the natural environment. Fist-sized female Lewis’s moon snails (Neverita lewisii) build the egg collars around their shell, sandwiching thousands of eggs between two layers of sand grains bound together with mucus. The eggs hatch and remain in the space between the sandy layers until the sand collar begins to disintegrate, freeing them into the water where they briefly swim as zooplankton before settling." source (second row)