A couple more views:
Some of you may have guessed this animal already. If not, be forewarned. The next image may give it away:
And here's another with the foot extended over the tip of the shell:
This is a Lewis's Moon Snail (Neverita lewisii, formerly Polinices lewisii or Euspira lewisii). Joe kindly let me photograph this small individual that he discovered locally. It was only ~2 cm across.
Moon snails occur on the tidal flats in Bodega Harbor, and occasionally in subtidal sandy areas offshore. They're active predators that feed on clams (by drilling a hole through the shell!).
The first few photos highlight the long tentacles, extended above the sand as the moon snail explored its surroundings. In the third photo, you can also see the siphon, outlined in black (in the upper right corner). Water is pulled into the siphon for respiration and sensing of the environment.
There's something about the design of moon snail shells that just draws you in.
Georgia O'Keeffe chose moon snails as subjects for several of her paintings. The San Diego Museum of Art owns one called Pink Shell with Seaweed. You can see an example of that painting here.