in H. G. A. Engler and K. Prantl, Nat. Pflanzenfam. 67[IV,3b]: 77. 1891.
Leaves opposite or whorled; petiole 0–0.5 mm; blade elliptic to oblanceolate, 3–6 × 1.2–3 mm. Pedicels 0.6–2 mm. Flowers cleistogamous; calyx irregularly symmetric, tube deeply lobed abaxially, 1.2–1.5 mm, tube 0.7–1 mm, lobes 0.5–0.6 mm; corolla strongly bilaterally symmetric, unilabiate or nearly so, 1.8–2 mm, longer than calyx, tube 0.3–0.5 mm, abaxial lobes 3, forming a prominent lip, 1.4–1.5 mm, lateral 0, adaxial (0–)0.4–0.6 mm. Capsules 1.1–1.3 × 0.9–1 mm. Seeds oblong to narrowly obconic, 0.3–0.4 × 0.2–0.3 mm.
Phenology: Flowering summer–fall.
Habitat: Fresh water tidal areas in sandy, gravelly substrates.
Elevation: 0–10 m.
Del., D.C., Md., N.J., N.Y., Pa., Va.
The range given for Micranthemum micranthemoides is historic; it is presumed extinct in the wild but may still be available in the aquarium plant trade. The most recently collected specimen examined by the author was gathered in 1941 by M. L. Fernald (Fernald & Long 13754, GH) along the Chickahominy River near Cypress Bank Landing, Virginia.
Micranthemum nuttallii A. Gray is an older, but illegitimate name that pertains here; it was an avowed substitute for Hemianthus micranthemoides.