Need help with ID of bivalve fossil with possible fossilized pearls

[YHWH]

I found this fossil in a mountain stream near Lovell, Wyoming. I was wondering what species it could be? It has a shell like some kind of clam. It also has 3 reddish brown stones in it that shimmer in the light. Could these be pearls? The rest of the fossil doesn’t have any reflective gleam in the light. I added blue arrows and circles to show where the possible pearls are at. The whole fossil is about 1 and 10/16 inch long x 1 and 1/4 inch wide x 3/4 inch in height (from the back of the shell to the tip of the highest pearl). Thanks!

[Darktooth]

This looks like the internal structure of a horn coral that has been broken open and water worn. There also appears to be some kind of mineral replacement.

[Tidgy's Dad]

Yes. +1 for solitary rugose horn coral. 

And cubic galena crystals, I think. Though some of them look more like flourite. Could be both! 

[marguy]

Horn coral, + cubic: it could be worn pyrite crystals