SEA SHELL ON THE SEA SHORE

I spotted this, about the size of my hand, wedged under some rocks today. I gave it a tug to see if would come out, but it wouldn't move. I still haven't identified it, so suggestions welcome.

I'd also taken what I thought was going to be a beautiful iridescent starling photo but when I looked on the screen the poor thing had lesions around its face which I think must be avian pox so I'm trying to find a site where I can report it where I don't have to first define the sort of garden it was spotted in, as it was perched on the sea wall. And talking of gardens, I came to the conclusion that's it's pointless going to a garden centre at the moment for anything that isn't some sort of Christmas decoration. I just want a quick and colourful winter flowering hanging basket to put by the front door, but unless it's something covered in glitter I'm going to have to wait until January for anything actually garden related.