Spherical limestone fossils. ID?

[tiggerd100]

I’m fairly certain these are some type of fossils. The broken ones have a clear round spot in the middle. Found in southeast Kansas. Any ideas?

 

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[StoneHunter]

Perhaps bryozoans?

 

[old digger]

I have found those here in Montana also, when they are broken they tend to show lines radiating outward from the center. A friend of mine and myself call the Indian marbles. They appear to be some sort of crystalization that occurred causing their formation. But, not absolute on what they my be called.