The Paleozoic rugosan corals can be either solitary or colonial. Although they may have originally had aragonitic skeletons, all are now calcite. Rugosa corals are distinguished from the other Paleozoic group, the Tabulata, by having dissepiments and well developed septa in addition to the tabulae. As shown in the accompanying figure (Figure 3.1), rugosan corals have six primary septa and new septa are added in only four of the resulting six spaces with none added in the remaining two spaces. This septal arrangement is well illustrated in the external mold where the septa are preserved as gaps.
For examples of solitary forms which typically exhibit a cone or horn morphology (hence the informal name "horn corals") examine these specimens, - and This specimen has excellent dissepiments visible where the epitheca is worn-away. For examples of colonial rugosans view this specimen.