The genus
Placopyrenium is characterised by its (growing on rocks)
thallus, which is thick and distinctly patterned, breaking into (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 , while the underside is black. The thallus lacks (root-like
hyphae) and the stipes are directly attached to the . 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 , meaning the cells are closely packed together. The areolae's base and sides are enveloped in a (blackened) cortical layer, providing durability and protection. In terms of reproductive structures,
Placopyrenium develops (fruiting bodies) within the thallus. These perithecia lack an (an outer layer or envelope). The
asci (spore-producing structures) within the perithecia are , meaning they are shaped like a club. The spores produced in these asci are arranged in two rows (), and are either
ellipsoidal or ovoid-oblong in shape. These spores are colourless and typically , although they are often uniseptate, meaning they have a single internal division (
septum). Additionally, the genus produces short, rod-shaped
conidia (asexual spores). ==Species==