What is a genus of simple, stony corals; -- so called because they are usually flat and circular, with radiating plates, like the gills of a mushroom. Some of them are eighteen inches in diameter called?

fungia — A genus of simple, stony corals; -- so called because they are usually flat and circular, with radiating plates, like the gills of a mushroom. Some of them are eighteen inches in diameter. (Webster's 1913.)