E-Flora BC: Electronic Atlas of the Flora of British Columbia

Species description:
Thallus minute; lobes averaging to less than 1 mm wide, occasionally cylindrical; true (i.e., basally constricted) isidia absent AND
Thallus entirely or largely consisting of dorsiventral lobes (check basal primary lobes) AND
Not associated with Peltigera venosa AND
Upper surface sometimes angular, but generally not at all wrinkled; thallus cellular (paraplectenchymatous) throughout AND
Thallus flattened/dorsiventral more or less throughout AND
Over trees, moss or soil AND
Lobes distinctly elongate, appressed throughout (except occasionally weakly ascending near lobe tips), averaging to less than 0.1 mm wide; over decaying wood or moss

Comments:
Leptogium subtile was subsumed under L. tenuissimum by Sierk (1965), though other authors (e.g., Santesson 1984) recognize it as a distinct species.