I can't get my camera to focus on the small things, so I have to just describe it...
It's smalll now, maybe half the size of a dime. The shell is pretty circular with points around the edges. It's not completely flat and has somewhat of a point in the middle, but it's a very shallow shell. I haven't been able to look underneath, and it hasn't left the rock I found it on first, but it does move around. I did some digging on the interweb and haven't been able to find the right picture.
I got this rock from Liverocks.com. Any ideas?