Plants less than 2.5 mm. Leaves broadly linear to lanceolate, 0.8-2.5 × 0.15-0.4 mm; margins serrulate to strongly serrate distal to the proximal third; apex slenderly acuminate, papillose; costa not always apparent at the base, often filling the acumen, percurrent; areolation compact distally; median laminal cells smooth or slightly papillose; distal laminal cells somewhat papillose. Capsule with very few stomates, mostly near the base. Spores spherical or reniform, 43-107 × 35-75 µm, orange-brown.

Capsules mature year around. Moist or drying disturbed soil; low to moderate elevations (0-1000 m); Ont., Que., Sask.; Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kan., Ky., La., Md., Mich., Minn., Miss., Mo., Neb., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., Tenn., Tex., Va., W.Va., Wis.; Europe (Germany); e Asia (Japan); Pacific Islands (New Zealand).

Variety papillosum has not been considered worthy of recognition (A. J. Grout 1928-1940; V. S. Bryan and L. E. Anderson 1957), a conclusion sustained by the present studies. It was described originally because of its narrower leaves and a strongly papillose calyptra, but both characters have been found to vary independently. Variety crassinervium shares several characters with Ephemerum sessile (Bruch & Schimper) Müller Hal., a relatively common species in Europe, the Mediterranean islands, and north Africa. It was reported as occurring in North America (W. S. Sullivant 1856), and plants labeled as such from 'central Ohio' were distributed by Sullivant and Lesquereux in 1856 as number 21 of their exsiccata series Musci Boreali-Americani. Grout commented that number 21 is not E. sessile, and I have found no North American plants that are convincingly E. sessile, as distinct from E. crassinervium.