What is it?

When I stumbled across this it looked familiar, like something I had read about … and after a quick search online, I ended up figuring out what this was.

It is the egg casing of a whelk snail!

To read more about it, check out this link. I’ve since found several more. The shape of the sac represents the type of whelk, if it’s a knobbed or channel type, for example, more common near me.