Mystery solved! And it wasn't actually that great a mystery. The egg cases below were laid by a drill snail, a voracious oyster predator.
The beautiful orange drill snail in the middle of the photo is native to the Pacific Northwest... the two drab-looking snails on either side of it are non-native Japanese drill snails.
Here's a better photo of Japanese drills. On an unrelated note: normally, after four or five hours of work on the beach—bent over at the waist in blinding summer sun or soaking winter rain—even the most ardent tideflat fan is happy to call it quits. And quitting-time comes whenever the incoming tide says so. Here's a sequence of pictures taken at twenty second intervals on an incoming tide:
[…] you find a shell with a single hole bored into it, it was probably eaten by a some kind of drill snail. European drill snails, Japanese drill snails, and native drill snails are all found on our farm. […]