Plants dark green to tan. Stems to 1.5 cm, central strand present. Stem leaves appressed and incurved when dry, patent to spreading-recurved and not keeled when moist, monomorphic, long-lanceolate, grooved adaxially along the costa, especially near leaf apex, 2-3 mm, base scarcely differentiated or short-oblong, margins narrowly recurved in proximal 1/3-1/2 of leaf, often broadly crenate (ca. 8-10 cells across), sinuose or weakly notched in distal 2/3-3/4 of leaf, apex narrowly acute or narrowly blunt and long-acuminate, somewhat fragile; costa percurrent or short-excurrent, not strongly spurred, not much widened or tapering, lacking an adaxial pad of cells, adaxial costal cells quadrate, ca. 4 cells wide at mid leaf, guide cells in 2 layers; basal laminal cells very weakly differentiated medially, walls thin to weakly thickened, quadrate to short-rectangular, not perforated; distal laminal cells 7-9 µm wide, 1:1, nearly smooth or papillae simple or 2-fid, 2-3 per lumen, lumens rounded-quadrate, walls thin or evenly thickened, weakly convex on both sides of lamina, distal 2/3-3/4 of the leaf margins 2-stratose with 1-2 rows of cells similar to the medial cells of the lamina. Specialized asexual reproduction apparently by fragile leaves. Sexual condition apparently dioicous; perigonia not seen; perichaetial leaves little different from the cauline or with wider bases. Seta 0.8-1.2 cm. Capsule 2-2.4 mm; peristome teeth 32, linear, weakly twisted or to 1/2 times, ca. 700-1000 µm. Spores 8-10(-13) µm. Distal laminal KOH reaction yellow-red.
Sporophyte maturity undetermined. Trunk and bases of trees, soil over rock; moderate elevations; B.C.; Calif.; Europe (Spain).
Didymodon eckeliae is an uncommon species known from a few collections in southern and central California, and very recently in coastal British Columbia. F. Puche has collected it in Spain (Alicante, Font Rojas). It is closely related to D. vinealis, sharing the weakly differentiated leaf base, transverse section of the costa deeply convex, and the presence of a short, deep groove with the appearance of a long-elliptical window on the adaxial surface of the costa near the apex, but is immediately distinguished by distal leaf margins 2-stratose in a narrow band extending to the broadened base and often scalloped to weakly notched, at least in leaves of sterile plants. Didymodon vinealis may occasionally have distal leaf margins 2-stratose in small patches, but never evenly so (though sometimes, apparently, completely 2-stratose in distal portion of leaf); D. nicholsonii has a much broader, long-ovate leaf outline. Didymodon sinuosus (Mitten) Delongne of Europe is similar but its immature leaves have denticulate apices, and the distal marginal leaf cells are minutely crenulate (as in Trichostomum tenuirostre) and not 2-stratose.