Plants wiry in appearance, 1--2 mm wide measured across leafy stem, to 15 cm long. Stems irregularly to regularly 1--2(--3)-pinnate, often somewhat frondose or bushy from arching innovations; branches slender. Stem leaves distant, erect-spreading to wide-spreading, broadly ovate to deltoid, only slightly concave, 0.8--2(--2.5) × 0.8--1.5 mm; margins spinose-serrate distally, serrulate at base; gradually tapered to apex; costa usually double, sometimes single, single and branched, or triple; median cells 30--65 × 4--7 µm. Branch leaves loosely imbricate, ovate, 0.4--1.2 × 0.3--0.8 mm; costa often single, but variable. Seta 20--45 mm. Capsule ovoid-ellipsoid, 1.5--2.5 mm.

Soil, humus, old logs, and rocks in moist forests, especially under spruce-fir; 0--2000m; B.C., N.B., Nfld., N.S., Ont., P.E.I., Que.; Alaska, Maine, Mass., Mich., N.H., N.Y., N.C., Tenn., Vt.; Eurasia.

In addition to the numerous highly branched paraphyllia, stems in Hylocomiastrum umbratum (but not H. pyrenaicum) also bear some scattered larger paraphyllia. These are more than twice the size of typical paraphyllia, are lanceolate-subulate with toothed margins, and closely resemble the foliose pseudoparaphyllia. Under a dissecting microscope these larger paraphyllia are clearly visible at 30× magnification scattered among the shorter paraphyllia. In otherwise difficult specimens, the presence of these larger foliose paraphyllia can be used to distinquish H. umbratum from H. pyrenaicum.