Pleurodictyum expansum

Pleurodictyum It is an extinct genus of tabulated corals, characterized by corallites polygonal. They are almost always colonial, forming colonies defined by a calcite skeleton and similar in appearance to a honeycomb. Colonies commonly encrust hard substrates such as rocks, shells, and hard carbonate bottoms. 

Tabulate corals (order Tabulata) are an extinct group of anthozoans (polyp phase only, no medusa phase) hexacoralarios (gastrovascular cavity divided into many chambers by septa or sarcosepts. These sarcosepts are usually in a number multiple of six and have one or two rows of tentacles, also in number a multiple of six.). 

lTabulate corals are characteristic of shallow waters from the Silurian to the Devonian. The water level rose in the Devonian, and tabular corals became much less common forms. They finally became extinct in the Permian-Triassic mass extinction event.