Wikipedia
Pleurocystites
Pleurocystites (meaning rib bag or side bladder) is a genus of rhombiferan echinoderm (a cystoid) that lived in the Late Ordovician. Its fossils are known from Europe and North America. Pleurocystites grew to a height of 2 centimeters (3/4 of an inch) and fed on tiny, floating particles.