An extinct class of small, disk-shaped echinozoans in which the lower surface of the body probably consisted of a suction cap for adhering to substrate, and the upper surface was covered by separate plates arranged in concentric rings. The mouth lay at the center of the upper surface, and the anus, also on the upper surface, lay some distance between the margin and the mouth. Little is known of the habits of cyclocystoids. They occur in rocks of middle Ordovician to middle Devonian age in Europe and North America.