(also Tetracorallia), a subclass of extinct coelenterates of the class Actinozoa that existed from the Middle Ordovician to the end of the Permian. The coelenterates were solitary or colonial polyps having a calcareous external skeleton. Solitary forms were hornlike, cylindrical, or prismatic. Some had an operculum that covered the osculum when there was danger. The polyps inhabited shallow zones of seas, attaching themselves to underwater objects or lying freely on the bottom. In contrast to extant hexactinal polyps, Rugosa did not form reefs. The coelenterates are valuable in understanding the stratigraphy of the Paleozoic and the total evolution of coral polyps.