A Shell-shaped Mystery

What on Earth is this thing? I found one whilst searching for fossils at Ashfield Brick-clay Pit in Conisbrough and could not figure it out. It could easily be mistaken for a bivalve mollusc shell (that was my first thought) but it does not appear to be fossilised, does not match the limestones it was near and does not look like a mollusc when looked at properly.

I hadn't looked at it since Easter until today when I found another, this time in my back garden. It is organic, but what is it? For all I know it could have been produced by either plant or animal. It appears to be able to close into two parts, so a seed of sorts perhaps? If anyone has any idea, no matter how crazy, I want to hear it?

Perhaps it is the shell of a grenade used by tiny people in my garden.....