Multiple Blastoid (Pentremites) Plate - Illinois
Blastoids are an extinct type of stemmed echinoderm related to crinoids. They would have attached themselves to the bottom of the ocean with an stalk and were filter feeders. The body of theca of blastoids resembles and acorn and is composed of interlocking armored plates. They first appeared in the Ordovician period, and reached peak diversity during the Mississipian before going extinct in the Permian extinction.