A Palaeozoic order of solitary and colonial corals which appears in the Ordovician and becomes extinct in the Permian. The corallum contains radial plates (septa) and horizontal plates (tabulae), and sometimes oblique plates (dissepiments). The septa developed in insertion cycles of four and many species retain a degree of bilateral symmetry. Solitary rugosans seem to have preferred soft substrates since they had no means of cementation and colonial forms seem to have relied for stability on the weight of the skeleton.