The genus Placopyrenium is characterised by its epilithic (growing on rocks) thallus, which is thick and distinctly patterned, breaking into areoles (small, island-like sections) or squarrose-areolate (with scale-like, spreading areoles). These areoles have a unique shape, being noticeably narrowed at their base and extending into elongated stipes (stalks), which are ashen in colour on the upper surface due to a protective epinecral layer, while the underside is black. The thallus lacks rhizohyphae (root-like hyphae) and the stipes are directly attached to the substrate.[2]
The entire thallus is composed of cells that are either angular or rounded, measuring 5–10 μm in diameter, forming a structure that is nearly paraplectenchymatous, meaning the cells are closely packed together. The areolae's base and sides are enveloped in a carbonaceous (blackened) cortical layer, providing durability and protection.[2]
In terms of reproductive structures, Placopyrenium develops perithecia (fruiting bodies) within the thallus. These perithecia lack an involucrellum (an outer layer or envelope). The asci (spore-producing structures) within the perithecia are clavate, meaning they are shaped like a club. The spores produced in these asci are arranged in two rows (biseriate), and are either ellipsoidal or ovoid-oblong in shape. These spores are colourless and typically simple, although they are often uniseptate, meaning they have a single internal division (septum). Additionally, the genus produces short, rod-shaped conidia (asexual spores).[2]
