Sand collars are the egg masses of certain sea snails, marine gastropod mollusks in the family Naticidae, sometimes known as "moon snails".
These egg masses often wash up on sandy beaches, either whole or sometimes in fragments. When they are whole, sand collars are shaped like an old-fashioned detachable shirt or blouse collar (hence the name). They consist of sand grains cemented together by a gelatinous matrix, with embedded eggs.
A fresh sand collar feels stiff and yet flexible, as if it were made out of plastic. Each sand collar contains thousands of capsules, each one housing one or several live embryos. In species with planktonic development, these embryos hatch out as bilobed veligers. After the eggs hatch, the sand collar disintegrates.