Plants with shoots 10 30 × 1.5 2.5 mm, prostrate, forming patches or thick mats, green to reddish brown; branches few. Stems stout, ca. 250 300 µm; branching terminal type (base of branch with a lanceolate half leaf) or occasionally lateral intercalary; cortical cells thin walled, 25 30 µm, medullary cells similar with slightly thicker walls. Leaves imbricate, widely spreading, ovate to rounded quadrate, 950 1100 × 950 1150 µm, the apex rounded to slightly retuse; leaf cell lumen rounded, sub quadrate to hexagonal, median cells 25 35 × 28 40 µm; marginal cells slightly smaller, ca. 22 -25 µm; cuticle slightly verruculose striolate near leaf base; cell walls thin, trigones with concave sides, not bulging; oil bodies 7 15 per cell, ovoid to ellipsoidal, 3 5 × 5 9 mu, finely granular. Underleaves absent or occasional and subulate near stem apex. Androecia terminal, spicate, eventually becoming intercalary; bracts in 4 6 pairs, imbricate, slightly smaller than leaves, the dorsal base ampliate with 1 2 inrolled teeth; antheridium one per bract; stalk 2 seriate. Gynoecia terminal on main shoot with 1 2 subfloral innovations; bracts larger than leaves, 1 1.2 × 0.9 1.2 mm, sheathing at base, spreading distally, rounded to retuse with 1 2 laciniate teeth or cilia on lateral margins, the base narrowly connate with bracteole; bracteole large, 0.3 0.4 mm with several laciniate ciliate lobes; perianth cylindrical to fusiform, 3 3.5 mm, distall portion 4 5 plicate, narrowed to laciniate ciliate mouth, cilia 5 10 cells long.

Humus rich soil, rock, logs or tree bases in moist temperate deciduous or Spruce fir forests; Alta; B.C. N.B., Nfld., N.S., Ont. Que.; Alaska, Conn, Fla., Idaho, Ill., Ind., Iowa., Kans., Ky., Maine, Md., Mass., Mich., Minn., Miss., Mo., N.H., N.J., N.Y., N.C., Ohio, Ore., Pa., R.I., S.C., Tenn., Vt., Va., Wash., W.Va., Wis., Wyo.; Mexico; Europe; Asia.

Minor varieties have been reported from single sites in the flora area. Jamesoniella autumnalis var. myriocarpa (Brinkman) Frye and L. Clark is an inadequately described plant from a site in Nova Scotia; the Japanese J. autumnalis var. nipponica (S. Hattori) S. Hattori was reported from a site in Virginia, and the more robust J. autumnalis var. heterostipa (A. Evans) Frye & L. Clark from Quebec. Jamesoniella autumnalis is subject to environmental variation and the differences on which these varieties are based are poorly understood. These plants are most likely environmental variants.