Unknown Thing....Probably Geological

[Omnomosaurus]

I've been digging through an old stash of fossils I collected years ago, and came across this thing. 

 

I seem to remember finding it about 20 years ago whilst out walking in North East Wales, UK.

 

It is quite lightweight for the size, and makes a very different sound compared to the dull thud of a rock, when tapped with a fingernail.

 

There also appears to be some sort of impression prominent on the "inside" curve.

 

I expect this to be something geological (or even archaeological), but on the off-chance anyone has an inkling what it might be, you'd be putting years of curiosity to rest!

 

 

 

 

 

 

Also, there are small features throughout the rock/specimen, with some parallel looking grooves amongst them: