Plants 3-10 mm, perennial, in dense short turfs, pale-green to yellow-green. Stems branched by several innovations from axils of stem leaves or outer perichaetial leaves, often becoming prostrate with age. Stem leaves erect-spreading, lanceolate to ovate-lanceolate, acuminate, 0.7-1.5 mm, becoming reduced proximally; costa percurrent to slightly excurrent; laminal margins smooth, weakly recurved; median and distal laminal cells rhomboidal to hexagonal, 3-4:1, 35-70 × 12-18 µm, proximal cells rectangular, 2-4:1, 40-70 × 18-24 µm, short-rectangular to quadrate in alar region in 2-6 rows extending 7-15 cells along proximal laminal margin; leaves of innovations similar to distal stem leaves except being smaller, gradually reduced proximally. Perichaetial leaves larger, 1.5-3 mm, elongate-triangular, lanceolate or ovate-lanceolate, gradually acuminate; costa percurrent to short-excurrent; laminal margins smooth, weakly recurved distally; median and distal laminal cells uniformly rhomboid to hexagonal, 3-7:1, 35-130 × 15-28 µm, proximal cells rectangular, 4-6:1, 70-140 × 15-28 µm, sometimes hyaline in alar region but cells not strongly differentiated. Specialized asexual reproduction absent. Sexual condition autoicous, antheridia terminating axillary leaf buds. Capsule terminal, 450-700 µm. Spores typically 20(8-36) per capsule, triangular to rounded-polyhedral, 140-250 µm, smooth to granulose, yellow.
Capsules mature early spring (Mar) or rarely late fall (Nov). Uncommon to rare on moist open sandy or loamy soil, along roadsides and in grassy often disturbed areas; 0-200 m; Fla., Ga., Tex.; Mexico; South America.
Archidium hallii can be distinguished from similar-looking forms of A. tenerrimum by autoicous sexuality and leafy bracts enclosing the antheridia.