Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.

Thallus: crustose, thin, dull, continuous to cracked or somewhat granulose upper surface: gray to brownish Perithecia: tiny (0.1 - 0.2 mm in diam.), almost globose, in hemispherical thalline warts; exciple: pale except for the apex involucrellum: covering upper third or half of perithecium asci: c. 40 x 15-20 µm ascospores: ellipsoidal to ovate, 1-septate, 10-15 x 4-6 µm Conidiomata: unknown Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on limestone on dry sites World distribution: with certainty known only from central Europe (Harada 1992) and Mexico Sonoran distribution: only known from one locality in Baja California Sur. Notes: Thelidium minimum is characterized by an epilithic thallus, a well-developed involucrellum, and small ascospores. Thelidium impressum (Stizenb.) Zsch., which has spores of similar size, differs in its endolithic thallus. Thelidium microsporum Lynge is an arctic species that almost lacks a thallus and has even smaller spores.