Maybe a bivalve?

[HeatherMF]

I think this is a bivalve. I'm curious as to whether or not I'm right, and what it's age might be. 

[Miocene_Mason]

I believe you are correct.

[izak_]

Yes, it is. We can't tell you the age without the location you found it (nearest town)

[JohnBrewer]

Yes, a bivalve. Do you know where it came from? That will help with dating it. I think @Herb might be able to help with identification. 

[HeatherMF]

Half Moon Bay, California.