Although this shell had washed up on the beach, Boreal Wentletraps (Opalia wroblewskyi, formerly Opalia borealis, formerly Opalia chacei) are often associated with their prey — Giant Green Anemones (Anthopleura xanthogrammica).
Here's another view, this time from below:
Boreal Wentletraps reach a maximum size of ~35 mm long, so keep an eye open for these beautiful turreted shells.
P.S. There's a bonus snail in these photos. Near the top edge, there are two very small reddish-brown snails. They're actually adult snails (that's about as big a they get!). Say hello to Barleeia haliotiphila.