Plants less than 2.5 mm, gregarious in abundant, persistent, matted protonemata. Leaves setaceous to linear-lanceolate, 1.1-0.23 × 0.12-0.2 mm; margins serrate to strongly spinose; spines 40-60 µm, spreading or recurved to 45°; or more, sometimes 2-celled; apex narrowly acuminate; costa occasionally absent in the proximal third, but usually strong, nearly ¹/3 of the base, percurrent or excurrent, spinulose or spinose; areolation firm proximally and denser distally; median laminal cells in vertical rows, papillose or occasionally smooth; distal laminal cells spinose. Capsule with columella resorbed before meiosis; stomates few, mostly in the proximal half. Spores spherical or reniform, 58-118 × 42-80 µm. Capsules maturing year around. Sides of ditches and ravines, moist paths, old fields, swamps, moist or drying soil in disturbed, partly sunny areas, occasionally on rotting wood; low to moderate elevations (0-700 m); Ont., Que., Sask.; Ala., Ark., Conn., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., Ohio, Pa., S.C., Tenn., Tex., Va., W.Va., Wis.; West Indies (Cuba), Central America (Honduras); South America (Brazil); Europe; Asia (China, Japan, Taiwan). Ephemerum spinulosum bears, as part of the abundant and persistent protonemata and rhizoids, red-brown, thick-walled structures. They also occur occasionally in E. serratum, but are only rarely seen in other Ephemera. The cells may be long-lived vegetative diaspores, possibly a drought tolerance mechanism (A. J. Grout 1928-1940; J. G. Duckett et al. 1993). As in all species of Ephemeraceae, E. spinulosum is polymorphous. Although rare, extreme expressions are found; e.g., leaves rather broadly linear, an uncommonly thin costa, the marginal dentation short-hardly more than the protruding distal ends of marginal cells, and laxer areolation. When such extremes occur in combination, the plants may approach E. crassinervium var. crassinervium, Micromitrium wrightii, or M. tenerum, but other characters point to the correct determination. This combination of traits exemplifies the nature of variation found in the Ephemeraceae.