The writer has collected the _Lecideaceae_, with other fungi, in Butler County for fifteen years, and has worked for the Ohio Biological Survey in Preble, Warren, Highland, Fairfield, Adams, Hocking, and Lake counties. Besides these collections made by the writer, a few specimens were examined from Champaign, Hamilton, Wayne, Morgan, Madison, Muskingum, Franklin, Vinton, and Summit counties. Of the 37 species treated in this paper, 24 had not been reported from Ohio previously. [Footnote A: Contributions from the Botanical Laboratory of Miami University.--XVIII] _Systematic Account._ LECIDEACEAE Thallus crustose, without plectenchymatous cortex (Fig. 2, a), varying from granulose and often evanescent to conspicuous, areolate, or even subsquamulose conditions, attached to the substratum by hyphal rhizoids (Fig. 2, d), and in a few instances extending up as a veil and surrounding the apothecia laterally, the hyphae densely interwoven toward the upper surface, but more loosely disposed below (Fig. 2, a and b); apothecia usually minute or small, commonly rounded, the exciple weak and obscure (Fig. 10, d), or more strongly developed when conspicuous and much darker in color (Fig. 11, b); hypothecium varying from hyaline to dark brown (Fig. 10, b and Fig. 11, c); hymenium almost always lighter and commonly hyaline (Figs. 10 and 11, a); paraphyses usually simple, but branched forms to be found frequently (Figs. 1 and 12), pale throughout or darkened toward the sometimes enlarged apex, commonly more or less coherent and indistinct at maturity; spores simple and hyaline to muriform and brown (Figs. 2, 3, 4, 5, 6, 7, 8, 9, and 13). KEY TO THE GENERA Spores minute, numerous in each ascus _Biatorella_, p. Spores larger, usually 8 in each ascus, Spores hyaline. Spores one-celled (simple) _Lecidea_, p. Spores more than one-celled (compound). Spores 2-celled _Biatorina_, p. Spores 4- to several-celled. Spores ellipsoid, fusiform, or dactyloid _Bilimbia_, p. Spores acicular _Bacidia_, p. Spores brown, or becoming brown. Spores 2-celled _Buellia_, p. Spores 4-celled and becoming muriform _Rhizocarpon_, p. Biatorella De Not. Giorn. Bot. Ital. 21. 192. 1846. Thallus granulose to verrucose and subareolate, sometimes inconspicuous and evanescent; apothecia minute to middle-sized, adnate or more or less immersed, exciple usually prominent and persistent, but sometimes becoming covered, disk flat to convex; hypothecium and hymenium pale to brown; spores simple, hyaline, minute, numerous in each ascus. KEY TO THE SPECIES OF BIATORELLA The whole apothecium dark colored 1. B. _simplex_ The disk of the apothecium white-pruinose 2. B. _pruinosa_ 1. Biatorella simplex (Dav.) Br. & Rostr. Bot. Tidssk. 3: 241 1869.