Calcite crystals inside a test of the cystoid <i>Echinosphaerites aurantium</i> (Middle Ordovician, Estonia).
<a href="//commons.wikimedia.org/wiki/User:Wilson44691" title="User:Wilson44691">Mark A. Wilson</a> (Department of Geology, The College of Wooster).<a rel="nofollow" class="external autonumber" href="http://www3.wooster.edu/geology/MWilson.html">[1]</a>
<span class="int-own-work" lang="en">Own work</span>