Thallus squamulose, olivaceous brown, forming flattened cushions, to 2 cm diam.; individual squamules to 1 mm broad, rounded or elongate, incised with notched, thickened margin, often with isidioid secondary lobules (Fig. 1A). Upper surface uneven, but without isidia. In section 80–120 µm thick with incomplete cellular cortices enclosing a medulla of loose hyphae with scattered short chains of the nostocoid photobiont; individual cells 4–6 µm diam.
Apothecia and pycnidia unknown. All chemical reactions negative; no secondary lichen substances (TLC).
Leptochidium crenatulum is easily distinguished from L. albociliatum (Fig. 1B) on its small-squamulose thallus with non-hairy lobe-margins, which instead are incised and notched, often with isidioid secondary squamules and with a non-isidiate upper surface. It is often thinner, more brittle and anatomically with fragmentary cellular cortices and no cellular inner structure as is so often found in Leptochidium albociliatum, which is a species mainly confined to the subalpine region, though occasionally descending to the coast in the northwest. That species may have rather condensed lobes which always have entire, somewhat undulating margins with short, glassy hairs (Fig. 1C), and these are rather easily separated from the more squamulose L. crenatulum. The Scandinavian material of L. albociliatum differs most distinctly from that illustrated by Brodo et al. (2001) from Pacific North America (see also Fig. 1D), in having smaller, thicker and narrower lobes with cellular internal parts, and in being mostly sterile. However, the original material from the French Alps (distributed as nr 233 in Desmazières exsiccate; specimen at UPS studied) closely resembles the Scandinavian ones, which ensures the use of that name for the hairy Scandinavian material (leaving the Pacific North American, as well as some similar European material in need of further study).
Mainly a species of mossy rocks by mountain rivers, though also occasionally found in drier sites. L. crenatulum is mainly found in the northern and alpine parts of Fennoscandia (Fig. 2). It is not surprising that it descends to the ocean in the northwest, a phenomenon already noticed by Wahlenberg (1812). The known localities outside Fennoscandia strongly support the impression of an arctic – alpine species, as they are situated in Iceland, Greenland, and Baffin Island in Canada.