Plants in dense cushions, turfs or scattered tufts to 3 cm high, dark green to olive or brownish green, sometimes nearly black. Leaves obscurely to clearly 3 ranked, not at all secund, moist plants with the erect-spreading leaves keeled along costa, dry plants with proximal portion of leaves erect to imbricated on stem and distal portion hamate to somewhat coiled and appearing rather crispate, oblong lanceolate to ligulate, to 3 mm long, less than 10: 1. Median laminal cells in regular rows, isodiametric or nearly so, to 12 µm broad, thin walled but with strong corner thickenings and rounded lumens, with cuticular papillae arranged in 2–3 parallel longitudinal streaks over both the lumens and the lateral cell walls.. Basal cells rectangular to linear in a small to large area, to 12 µm broad, to 5: 1, mostly with thin lateral walls but thick end walls, mostly smooth but occasionally with light cuticular papillosity. Costa filling about 1/5 of the leaf base, without a well-defined adaxial groove, ending within a few cells of the leaf apex. Leaf apex acute to blunt, without an elongate terminal cell. Abaxial cells of costa linear and thick-walled, to 8 µm broad, to 6: 1, smooth. Adaxial cells of costa rectangular, to 15: 1, to 10 µm broad, rather thin-walled and smooth. Costa cross-section at mid-leaf with a single layer of guide cells and with substereid cells ad- and abaxial to those guide cells. Margin entire, recurved especially near middle. Axillary hairs more than 10 celled, 300–400 µm long, with one basal brown cell. Rhizoids red-brown, smooth, to 8 µm in diameter at base, inserted along the adaxial line of leaf insertion, Stem triangular in section, without central strand, with thin-walled inner cortical cells and with 1–2 rows of outer thick walled cortical cells; hyaloderm not present.
Autoicous with one to several perigonia in leaf axils below the terminal perichaetium. Perichaetial bracts erect, completely sheathing the setae, truncate and apiculate with most of their laminal cells elongate and smooth. Seta yellow, to 2 mm long, straight. Capsule erect and symmetrical with the urn elliptical, wide-mouthed, to 1 mm long, about 2: 1, regularly sulcate. Operculum inclined rostrate, to 1/4 of urn length. Annulus not differentiated. Peristome absent. Exothecial cells in regular rows, rectangular, 2–6: 1, mostly 20–25 µm long; cells of the capsule ridges with very thick lateral walls on ridges but those in the valleys of the sulcate capsule having thin walls. Stomates phaneroporous, restricted to neck. Spores smooth, to 12 µm in diameter.
This is a plant primarily of high elevation, mostly timberline sites in California. I have, however, observed it at several near sea level localities where it grows as depauperate clumps on serpentine rocks. Amphidium lapponicum in California is usually rather easy of recognition. It is a smaller and darker plant than A. californicum, and the crispate leaves may be hamate at the apices but those apices will not show any significant amount of circinate coiling. Microscopic features of A. lapponicum allowing distinction from A. californicum include the nearly plane leaf margins and the papillae which, while arranged in rows, appear as discrete and rounded structures rather than cuticular ridges.