cephalodium (noun, pl. cephalodia) – small (to ca. 0.5-1 mm), delimited, gall-like thallus structure containing a secondary photobiont, usually a cyanobacterium, on or within the main thallus containing the primary (usually algal) photobiont; found in diverse genera, including Peltigera, Lobaria, and Stereocaulon; forming either as distinctly delimited, sometimes elevated and usually darkened patch within the thallus surface, or growing into a distinctly shaped surface structure, i. e., a warty, squamulose, or shrubby outgrowth from the thallus surface. This is:part of upper thallus surface. Kind of:upper surface structure.