Stems to 2.5[-7] cm, sclerodermis present. Leaves ovate, ligulate, elliptic or short-lanceolate, (0.3-)1-2(-2.5) mm, entire; leaf base usually not differentiated in shape but occasionally widened and ovate; costa ending 1-3 cells before the leaf apex (blunt-leaved plants) or percurrent or short-excurrent as a stout mucro, rounded to semicircular in section at mid leaf, guide cells 2-4; distal laminal cells usually subquadrate, occasionally triangular or transversely elongated, 1:1-2; often 2-stratose marginally or in patches medially.
Limestone, occasionally conglomerate, sandstone, basalt, clay, soil or siliceous rock, cliff faces, gorge walls, boulders, streamsides, seepage and humid places; warm temperate areas to Arctic tundra; low to high elevations (100-2000 m); Greenland; Alta., B.C., Nfld. and Labr. (Nfld.), N.W.T., Nunavut, Ont., Yukon; Alaska, Ariz., Ark., Colo., Ill., N.Mex., Okla., Tex.; Mexico; Central America; South America; Eurasia.
The perichaetial leaves of Molendoa sendtneriana reach 1.7 mm, little more than half the length of those of M. hornschuchiana. The phenomenal range of variation in leaf length and shape matches that of Dichodontium pellucidum (Dicranaceae). Because M. sendtneriana has the appearance of a Didymodon, it is important to examine the collection carefully for the distinctive lateral perichaetia (terminal in Didymodon). Both M. sendtneriana and Anoectangium aestivum have the distinctive lateral perichaetia, but large plants of the former have two stereid bands, and small plants have a costa ending below the apex and also often have leaves with 2-stratose portions, features that are absent from the latter in the flora area. The most distal leaves of M. sendtneriana are commonly a deep green and constrast strongly with the tan proximal portions of the turf. The lateral perichaetia, when present, will distinguish this species from Hymenostylium recurvirostrum, which grows commonly on the same substrate; when absent, the commonly elongate distal leaf cells in the leaf mid portion and the lack of an adaxial costal epidermis of the latter species (in the flora area) are helpful distinguishing traits. When sterile, Gymnostomum aeruginosum is similar to M. sendtnerianum but has smaller laminal papillae, mostly 3-5 per cell, and smaller leaf cells. Often plants of M. sendtneriana are encrusted with a distinctive black substance distally. Variation in leaf size and shape is substantial (see R. H. Zander 1977); usually the leaf apices of large plants are acute and those of small plants are obtuse.