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==