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, broadly crenate (ca. 8--10 cells across) in distal 2/3--3/4 of leaf, apex narrowly acute and long-acuminate, fragile; costa 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 midleaf, 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 by fragile leaves. Perichaetial leaves little different from the cauline, archegonia present, antheridia absent. Sporophyte unknown. Distal laminal KOH reaction red.

Sporophyte maturity undetermined. Trunk of Quercus agrifolia; moderate elevation; Calif.

This endemic species is known only from the type locality: Calif., San Diego Co., 13 km NE of Lakeside, Barona [Rancheria] Indian Reservation, I. L. Wiggins, April 9, 1954 (holotype--NY), mixed with Grimmia pulvinata (Hedwig) Smith & J. C. Sowerby. Didymodon eckeliae is closely related to Didymodon 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 the scalloped distal leaf margins. Didymodon vinealis may occasionally have distal leaf margins 2-stratose in small patches, but never evenly so throughout the leaf distal to the broadened leaf base; D. nicholsonii may occasionally sport narrowly acute leaf apices, but the leaves are not fragile and their margins are smooth. Didymodon sinuosus 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.