Dear Ichthyologists,
This was sent to me by Bex Seely of the Marine Biological Association. I think it is a ray scute but it was found on the shore of Llyn Cellwyn, a reservoir in Snowdonia.
http://en.wikipedia.org/wiki/Llyn_Celyn

The fact that it is a built reservoir makes mw wonder if this could have come up in some sort of building material. I've also known people to drop things like this for a laugh.

Post's pictures

Strange thing shore of Llyn Cwellyn image1 larger.jpg, 180.41 kb, 1024 x 768

"Hope is a duty from which palaeontologists are exempt."
David Quammen

Looks like a Thornback Ray scute to me.