Is anybody able to give me any information, such as age, of these fossils?

[Pleuromya]

Hello, I found these, I'm guessing bivalves, and was just wondering if anybody could tell me a little about them?

Sorry there's no scale, the larger one is about 2cm, and the smaller about half a centimetre. I found them in Northamptonshire, UK.

Thanks.

 

[Pleuromya]

I annoyed myself, as the missing chunk of the smaller shell was accidentally washed off by me.