Corroded shells of Margaritifera margaritifera Calcareous layers dissolved by acidic water, periostracum is what is left. Specimens from the River Erlau (1968), southeastern Germany.
<a href="//commons.wikimedia.org/wiki/User:PalaeoMal" class="mw-redirect" title="User:PalaeoMal">PalaeoMal</a>
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<span class="int-own-work" lang="en">Own work</span>