Last week when the tides were low I took a stroll way, way out on the mudflats. Lots of sand collars there—these are the egg cases of the moon snail, Euspira lewisii. When the snail produces the eggs in a mucous mixture sand is incorporated into its composition, producing these rubber-like rings.
Also found a couple of shells. No signs of actual live snails, though I’ve found them on other local beaches from time to time.
This webpage has a lot of interesting details.
One of the things I found in reading on the subject is that they can indeed pull their entire bodies inside their shells if they really need to—quite unbelievable when you see how big the extruded foot is in relation to the shell!