Unique among the Rugosa are specialized cyst-like structures in corals from an upper Carboniferous limestone within the Baird Formation in the Klamath Mountains, northern California. These structures, here referred to as septal cysts, occur mostly along the distal margins of the dark line extending along the axes of the major septa as seen in transverse section. However, they also commonly extend beyond the distal extent of those lines and may interrupt the fibrous coating in the more proximal parts of some septa. Their function is uncertain. Also present are small dissepiments which form a ring around the distal margins of the minor septa. These structures, however, do not appear to be related to the development of those septa. Some other taxa, including corals from the Bashkirian of Spain and the Kasimovian of Kansas, possess some specialized structures similar to those in the California specimens suggesting at least a remote relationship.