PENTREMITES

extinct genus of stemmed, immobile echinoderms (forms related to the starfish) abundant as marine fossils in rocks of the Carboniferous Period (from 360 to 286 million years ago), especially those in the midcontinent region of North America. The genus is mainly restricted to the Early Carboniferous Period (360 to 320 million years ago); more than 80 species are known. Specimens are frequently well preserved, allowing detailed anatomical and evolutionary studies.