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Blastoid

Blastoids, commonly called "sea buds," were a subclass of the class Cystoidea in the phylum Echinodermata. They dawned in the Ordovician, flourished during the Mississippian, and died out toward the end of the Permian period. A protective calcium carbonate shell surrounded the blastoid's head, or theca, and a plated stalk connected it to the sea floor.
Courtesy of Kentucky Paleontological Society