I'm sure I've seen something like this before, but there's a fossil-looking thing on the back.

[Pleuromya]

Hello, I found this rock, and it has this dipped area with a kind of pattern (I think it could be a concretion), and there's this fossil-looking thing on the back.

Is anybody able to ID this? Thanks.

 

[Pleuromya]

Here's the fossil-like bit.