Does anybody know what this could be?

[Pleuromya]

Hello, I noticed this 'thing' embedded in rock, does anybody know what it could be? Found in Northamptonshire, UK.

Thanks.

 

 

 

Ahh, I just broke it accidentally when moving it. Here's a photo of the inside of it:

[Ruger9a]

Chris, better photos are needed and some close up shots would help.

[Pleuromya]

5 minutes ago, Ruger9a said:

Chris, better photos are needed and some close up shots would help.

Ok, I'll do that now. 

[Pleuromya]

13 minutes ago, Ruger9a said:

Chris, better photos are needed and some close up shots would help.

I have a few more photos after this.

[Pleuromya]

Sorry if the photos still aren't great.

 

This is the bit that fell off.

[Pterygotus]

Still can’t really tell but my guess would be a poorly preserved bivalve .

[Pleuromya]

21 minutes ago, Pterygotus said:

Still can’t really tell but my guess would be a poorly preserved bivalve .

Thanks, I forgot to mention there were many bivalves nearby. You're probably right. 

[Darbi]

Bivalve was the first thing crossed my mind when seeing these pictures. Yeah, I'm with Pterygotus.