Thelopsis is a genus of lichen-forming fungi in the family Gyalectaceae. The genus was established by the Finnish lichenologist William Nylander in 1855 and contains small bark-dwelling crustose lichens that form thin crusts on surfaces. These lichens make flask-shaped fruiting bodies called perithecia, which contain numerous small ascospores divided by cross-walls. Recent molecular studies have revealed that the genus forms a closely related group within the broader Gyalecta complex, leading to taxonomic revisions that now recognise about a dozen species worldwide.

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