Plants scattered or gregarious in persistent, but not usually abun-dant protonemata, pale green. Stems 0.2-3.7 mm. Leaves erect, lanceolate, often with a broad base, narrowly acuminate, 0.85-2 × 0.27-0.4 mm, margins usually ± incurved beyond the middle, entire or nearly so. Sexual condition synoicous. Capsule dehiscing by a ring of differentiated cells at or distal to the equator, red-brown or red-black, globose or slightly flattened, slightly apiculate; exothecial cells in 1 layer, stomates none. Spores fewer than 100; various shapes, proximal face only rarely concave, 30-40 × 23-33 µm, dark red or dark brown.
Capsules mature summer-winter. Drying or dried ponds, edges of lakes or streams, bare soil in open forests; low to moderate elevations (10-500 m); Fla., Ga., Iowa, Ky., La., Mo., N.J., N.Y., N.C., Ohio, Oreg., Pa.; e Asia (Japan).
Calyptras in Micromitrium synoicum often are acentric, and as many as three capsules may mature within one perichaetium. Mature plants may persist with the capsule or capsules surrounded by only the bases of the leaves, the distal portions apparently eroded away.