Plants 2--20 mm, perennial, sometimes gregarious but mostly in dense short turfs, green to yellow-green. Stems variable, simple, with a few innovations, or often short and multi-branched by numerous innovations from axils of stem leaves or exterior perichaetial leaves, often becoming prostrate with age, innovations fragile and often detached. Stem leaves erect to erect-spreading, lanceolate, triangular or ovate-lanceolate, narrowly acuminate to subulate, 0.5--2 mm, becoming reduced below; costa percurrent to distinctly excurrent in hairpoint; laminal margins plane, smooth or finely serrulate distally; median laminal cells rhomboidal to prosenchymatous, 4--8:1, 45--90 × 9--15 µm, somewhat shorter distally and along margin, proximal cells short-rectangular to rectangular, 2--4:1, 25--60 × 12--16 µm, short-rectangular to quadrate in alar region in 2--6 rows extending 6--15 cells along proximal lamina margin; leaves of innovations similar to distal stem leaves except smaller, often sharply reduced below. Perichaetial leaves variable in shape, ovate, oblong, ovate-lanceolate to lanceolate, larger, 1--2 mm, short- to long-acuminate; costa percurrent to long-excurrent, often forming pellucid hairpoint; laminal margins smooth, plane or rarely weakly recurved; median laminal cells rhomboidal, linear-rhomboidal to prosenchymatous, 4--8:1, 45--140 × 9--15 µm, shorter distally and along margin, proximal cells hyaline, lax, rectangular to rhomboidal, 4--6:1, 50--90 × 12--18 µm. Specialized asexual reproduction lacking. Sexual condition autoicous, antheridia terminating axillary leafy buds. Capsule lateral, or both lateral and terminal, 270--600 µm. Spores typically 16 (4--60) per capsule, rounded-triangular to polyhedral, 110--320 µm, pale yellow, smooth to papillose.

Capsules mature fall through spring (Oct.--May). Common on moist open sandy, loamy, or clay soils, mud, and rock outcrops in a wide variety of open habitats, including roadsides, ditches, meadows, prairie, sand dunes, and creeks; 0--600 m. Ala., Ark., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Mich., Minn., Miss., Mo., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Va.; Mexico; West Indies; Africa; Asia; Indian Ocean Islands; Pacific Islands.

Archidium ohioense is the most widespread and variable species in the genus. J. A. Snider (1975) assigned many names to the synonymy of this species. In the U.S., the plants vary from rather tall, lax, weakly branched forms with longer leaves in wet sites to short, strongly branched forms with shorter leaves in drier sites. The perichaetial position is also highly variable. Future work may show the species to consist of a series of closely similar species, as suggested by the rather anomalous and scattered world distribution. Archidum ohioense is most easily confused with A. alternifolium and A. tenerrimum, but can be distinguished from them by the combination of primarily lateral capsules, antheridia enclosed in leafy bracts, and autoicous sexuality.