tiny shellfish in my pond

[aquilegia]

On Saturday I cleared a bit of duckweed and stuff out of my pond. On Sunday I moved the pile to the compost heap and found a little, empty shell inside it.

It was a hinged one, very pale/almost white and shaped rather like this: http://www.seashells.org/seashells/imperialvenus.htm

It was about 1cm wide.

What might it have been?

I can't find pictures of pond shellfish anywhere on the net.

My garden is a long way from the sea!

[aquilegia]