Plants 2--10 mm, perennial, in dense turfs, yellow-green. Stems simple or usually branched by several sterile innovations, becoming prostrate with age. Stem leaves erect to erect-spreading, lanceolate to narrowly ovate-lanceolate, acuminate, 1--2.5 mm; costa percurrent or short-excurrent, sometimes somewhat subulate; laminal margins recurved, smooth or finely serrulate distally; median and distal laminal cells irregular in shape, mixed quadrate, trapezoidal, short-rectangular, short rhomboidal to long-rhomboidal, 14--75 × 8--13 µm, proximal cells quadrate to short-rectangular, similar to cells above; leaves of innovations similar to stem leaves, not much reduced below. Perichaetial leaves broadly ovate-lanceolate, narrowly acuminate to subulate, sometimes flexuose to secund, same size as stem leaves; costa short to long excurrent, hairpoint pellucid, sometimes finely spinose; laminal margins recurved, smooth to finely serrulate; distal and median laminal cells similar to stem leaves, proximal laminal cells lax, wider, rectangular, 2--4:1, 35--115 × 12--25 µm, sometimes somewhat hyaline. Specialized asexual gemmae lacking. Sexual condition autoicous; antheridia terminating axillary leafy buds. Capsule terminal, 425--750 µm. Spores typically 28 (4--60) per capsule, angular to irregularly polyhedral, 120--230 µm, smooth or finely granulose, yellow to orange-r/ed.

Capsules mature early spring (Mar.), more rarely in autumn or winter (Oct.--Dec.). Uncommon on moist to dry soil along roadsides, in fields, rarely on rock; 0--250 m. Ill., Md., Okla., N.C., S.C., Tex., Va.; Mexico.

Archidum donnellii is a representative of a primarily southern hemisphere section of the genus. It can usually be distinguished from other species by the irregular areolation in the median and distal portions of the lamina, proximal lamina cells not strongly differentiated, and the autoicous sexuality. It tends to grow in drier habitats than do other species of Archidium.