A class of extinct Echinozoa which arose early in Cambrian times and died out before the end of the Carboniferous Period. They resemble the other Echinozoa in having the ambulacral radial areas bordered by tube feet which emerge through pores in or between the ambulacral plates. In this feature they differ from Crinozoa, to which they were once thought to be related. They differ from extant Echinozoa in having the mouth and anus on the upper side of the theca. Hinged ambulacral plates bordered the groove on either side and could be erected over it to convert it into a tube (see illus.). The theca shows strong pentamerous symmetry and is more or less discoidal; it bears no stem. The animals in some cases were cemented to the substrate by the lower surface; in other cases they were free and rested upon the sea bed. Brachioles are lacking. About 30 genera have been described and grouped into more than a dozen families.