Fossil on Barrassy beach

[Jaxfossils]

I found this fossil on Barrassy beach in Ayrshire Scotland. It looks like a shell on the outside with a mesh of white and grey rock inside. I haven't seen any other shell fossils that look like this though. It's partially burnt on the top, I found it next to a fire pit so it has probably been scorched recently. What do you think this is and how old do you think it is?

Edited July 3, 2021 by Jaxfossils

[Ludwigia]

Yes, it appears to be a fossilized bivalve. What you are seeing is the hardened grey and white sediment which found its way inside the shell after the creature dissipated. If there were no shell left, then this would be called a steinkern, or internal cast.