I feel lucky to have finally discovered one of these – it’s a sand
collar, or more accurately, the egg mass of a moon snail. "Moon snail"
is the common name for a family of gastropods known as Naticidae. The
snail uses sand and it’s own mucus to make these collars, which consist
of two layers. The eggs are between the layers. It’s pretty sturdy
until the babies start to hatch; then, it just disintegrates. This
means that no one should really have a sand collar in their
beach-combing collection, for if it’s intact, that means it was still
carrying babies. If you find one, by all means examine it, but then
leave it where it is so the eggs can hatch.
sand collar from Bunch Beach in Fort Myers, Florida