extinct genus of stemmed, immobile echinoderms (forms related to the starfish) abundant as marine fossils in rocks of the Carboniferous Period (from 359 million to 299 million years ago), especially those in the midcontinent region of North America. The genus is mainly restricted to the Early Carboniferous Period (359 million to 318 million years ago); more than 80 species are known. Specimens are frequently well preserved, allowing detailed anatomical and evolutionary studies.