thorium: a soft silvery-white tetravalent radioactive metallic element; isotope 232 is used as a power source in nuclear reactors; occurs in thorite and in monazite sands
metallic element: any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc.
bryum: type genus of the Bryaceae: mosses distinguished by mostly erect and tufted gametophytes and symmetrical short-necked capsules