I've been wondering for some time about this thing for some while now:
Most of the fossil is not exposed, but it seems to have a hemispherical shape. It does not seem like any kind of bivalve that I am aware of. What could it be?
Age: Maastrichtean
Locality: Čerevićki potok formation
Environment: shallow marine Sphaerulites radiolitid rudist thicket(likely somewhat protected waters)