Is this a fossil bi-valve and what genus/species would it be? (Llantwit Major, South Wales)

[HarryFossils]

Any help appreciated, thanks! :)

[Fossildude19]

This looks to be a steinkern, or internal cast.

Probably not enough detail there to confidently ID to species.  

Maybe @TqB would have an idea

[HarryFossils]

@Fossildude19Thanks for the reply! I looked at it a bit further and I believe it may be an internal cast as I can’t see many details on the outside of the specimen.  

[mighty micraster]

Cardinia?

[TqB]

Pleuromya is one possibility but hard to tell from an internal mould.

[HarryFossils]

@TqBI think you are right as it looks very similar to Pleuromya but as you have said it is very difficult to identify due to it being an internal cast