Thallus usually small, sometimes forming colonies to 5 cm diam. or more, foliose, rather thin, deeply lobed, often forming rosettes, slightly swollen when moist. Lobes 0.5-6 mm wide, often rather few, almost ear-like, ear-like, ± rounded at the apices, concave, frequently overlapping, often with ± ascending, wavy and sometimes convoluted margins. Margins entire or crenate, occasionally somewhat lacerate, but never swollen. Upper surface olive-green-brown to black, smooth or isidiate. Lower surface often with white rhizines, sometimes forming ± scattered hapters. Isidia at first globular, soon flattened and squamule-like.
Anamorph: information not available.
Teleomorph: ascomata apothecia, infrequent, appressed. Disc 1-2(-2.5) mm diam., flat. Thalline exciple often bearing lobules. Ascospores 26-34 × 13-15 μm, subellipsoidal or ± ovoid with rounded ends, 3-septate or submuriform, rarely 4- or 5-septate.
Assessed by Woods & Coppins (2012) as of Least Concern. The species is common and widely distributed.
Distinguished by the numerous, ear-like, often overlapping, lobes and laminal, flattened, squamule-like isidia which can resemble small lobes. The isidia are sometimes so numerous that they obscure the thallus. In very shaded habitats the thallus can become glaucous-emerald-green tinged.
Collema crispum var. metzleri (Arnold) Degel. (1954) BLS0441 (the combination in Blenothallia has not been made) is a diminutive morph, to 3 mm diam., with an almost crustose, non-pulpy thallus of scattered, ± overlapping lobes to 0.5 mm wide. Apothecia are few, solitary and small, 0.5-1.0 mm diam. It is chiefly associated with shaded chalk outcrops, and intermediates with B. crispum are frequent. Its occurrence and evolutionary position need further study.
Throughout the British Isles.
On calcareous rocks and walls, particularly on old crumbling mortar, often in damp, shaded places, more rarely on calcareous soils and shell-sand.