What fossil is this?

[Aurelius]

Not all fossil shells are 'rock' per-se. Many are changed by the surrounding minerals, but still contain the structure, and some of the original material, of the shell.  Many ammonites are preserved with their shells intact. I often dig up fossil snails that are just shells. No rock infill at all, just shells that look like they could be a week old.

[fossil0]

OK, I understand now.

 @FossilDawg said something about it being from the Upper Cretaceous (around 105 million - 66 million years). Can anyone confirm this?

Are Mactras extinct?

[abyssunder]

It could be Cretaceous, but I couldn't confirm that.
Mactra is not extinct.