A Walk on the Beach

This morning I was here.
It is supposed to be summer, warm, dry -- because this is Western Australia, and we were walking along a beach in Mandurah, which is where we intend one day to retire...

And it could have been a West Australian winter. Wet, blustery...intermittent showers and sun.

And my husband found this in the sand at the edge of the waves:

A piece of sponge...with attached mystery object, stuck to the stem.

That's an Australian 50c coin for size comparison.

The layers of "the thing" were hinged at the side where it was attached to the sponge.
So what does one do? Turn to the internet of course. Type in sea "egg case" and see what comes up. That led to pictures of whelk (conch) egg cases which seemed similar, and eventually to the Syrinx aruanus, or giant whelk...the biggest gastropod of all. According to one account, they can grow to 91cm long. That's 3 school rulers in length. Wow. When the snails hatch, they are 2 cms long.