Echinosphaerites aurantium from the Ordovician of northern Estonia.
This cystoid is preserved as an internal mold, but there are encrusters (notably
the bryozoan Scenellopora socialis) directly encrusting the outer
surface. Such preservation may be evidence for early calcite cementation inside
the cystoid theca. See Palmer, T.J. and Wilson, M.A. 2004. Calcite
precipitation and dissolution of biogenic aragonite in shallow Ordovician
calcite seas. Lethaia 37: 417-427.