Boring Clam

You've probably seen holes in the rocks on the Southern California beaches. Here's the shell of the critter responsible for that. Boring clams start out very small near the surface of the stone and continue to twist and grow over the years. This one is about an inch across and a couple from top to bottom. You can see on the upside the cutters in the outer shell that provide the boring process. That makes the hole bigger and evacuates the sand towards the top of the hole. In this picture, the fine sand grains on the left side are the mudstone. The grainy sand is beach sand on the right side.

Done