Okay, here's a species that I don't know much about, but am too intrigued by not to share.
This is Williamia peltoides, a marine pulmonate. Most pulmonates are air-breathers (e.g., land snails and slugs), but Williamia lives in the subtidal zone. I've occasionally found Williamia shells washed up on the beach, but this year I've encountered a few live individuals.
They look very similar to limpets, and are in a family that is sometimes referred to as "false limpets." The photo above shows their limpet-like shape. Note the pretty purple-red coloration with radiating white stripes.
They crawl on very large foot (see next photo).
In the photo above, you can also see a few interesting features. Note that they don't really have obvious tentacles (as do true limpets). Instead, they have rounded tentacular lobes. I believe that little black spot surrounded by white on the right side of the head is an eye.
Behind the head, on the right side, you'll notice two other mysterious structures (see close-up below).
The leftmost structure with several layers or lamellae (that makes it look corrugated) is a gill-like organ. It's not a true gill, and this feature separates them from true limpets (and is even unusual for pulmonates). The rightmost structure is rounded and flap-like. From what I can determine, this is an anal lobe and contains the anal opening. The undulating purple feature (with white spots) is the side of the foot.
A couple of other interesting notes about Williamia: They have a thin periostracum (a chitinous layer covering the shell) that often extends beyond the edge of the shell (unfortunately, it's not visible in these photos). And they release a white mucous when disturbed to deter predators. [Trimusculus, another marine pulmonate on Bodega Head, also releases toxic mucous that can paralyze sea star tubefeet!]
Williamia peltoides on Bodega Head, 7 July 2012.