I've just uploaded two images by Ralph Bottrill of a crystal cluster
inside marl that is littered with fragments of early Ordovician
trilobites. I found the piece in early January at Adamsfield in
Tasmania. The crystals were probably pyrite or calcite now replaced by
goethite. The matrix has dozens of trilobite fragments but the
negative mould highlighted appears to be a cephalon from the genus
Hystricurus, about 1 cm wide.
Regards
John Newlands