Thelotrema is a genus of lichen-forming fungi in the family Graphidaceae, the family to which all taxa in the former Thelotremataceae now belong. Members of the genus Thelotrema are commonly called barnacle lichens.

Description

Thelotrema lichens have a thallus with colours ranging from white to yellow-grey or light olive. The texture of the thallus can be smooth, uneven, or, with the presence of either an surface or a loosely to rarely dense to cortex. The and medulla frequently contain clusters of calcium oxalate crystals. The apothecia can be immersed or sessile and appear rounded or angular-rounded. The is partially covered by remnants of the, while the margin can be entire, undulate, or fissured, displaying a distinct double margin. The is absent in this genus. The excipulum is paraplectenchymatous,, and varies in colour from colourless to brown. It has distinct, and the are unbranched. Thelotrema are transversely septate to, fusiform-ellipsoid to oblong-cylindrical, and feature thin to thick septa with angular to rounded. The outer wall of the spores is often thick, and the spores may be colourless or brown. They can display weak to strong violet-blue amyloid reactions. The secondary chemistry of Thelotrema lichens generally includes stictic acid or norstictic acid or closely related compounds, with hypoprotocetraric acid appearing rarely.

Species

, Species Fungorum (in the Catalogue of Life) accepts 165 species of Thelotrema.