I have no idea what this is, maybe some sort of coral?

[SWMOnaturalist]

It reminds me of a soccer ball but very small. I was thinking some sort of coral that I haven't seen yet in my research. (I am new to this but I did try to research)

[daves64]

Looks like an internal mold of a crinoid calyx.

[Tidgy's Dad]

I think it's a favositid coral. 

[TqB]

I agree with internal crinoid calyx mould. You can see where the calcite has been dissolved out, and the plates in the middle here show concentric ridged ornament.