Lecidea lygommella forms a crust-like
thallus that can spread up to 7 cm wide and is relatively thin, ranging from 0.2 to 0.7 mm in thickness. The upper surface of the thallus varies in colour from whitish and pale grey to a rusty red-brown. This surface is , with irregular to angular measuring 0.5–1.0 mm across. These areoles can be flat and have a roughened or smooth texture. The , or outer layer, of the thallus is 20–50
μm thick, with its upper cell layer varying from unpigmented to grey-brown. Below the cortex lies a , about 70–100 μm thick, comprising
green algal cells each about 9–16 μm wide. The
medulla, a deeper layer of the thallus, does not react to
staining with
iodine (I−). The , which is the base of the lichen, can be dark grey and sometimes visible at the thallus margin, but often it is not developed. The
apothecia, or fruiting bodies of the lichen, are scattered or crowded. They are rounded to angular in shape, and range from being slightly embedded in the thallus to sitting on top of it, spanning 0.5–2.3 mm in width. The apothecial are black, flat to slightly convex, and either or coated with a rusty, red-brown (powdery coating). The margins of these discs are the same colour as the discs and are initially distinct, but may become less so as the lichen ages, sometimes acquiring a shiny appearance. The , the tissue layer beneath the
hymenium, is dark brown to black-brown at the base and turns violet-brown when treated with potassium hydroxide (K+). It is 120–250 μm thick. The hymenium itself is colourless, measuring 60–85 μm in height, and turns blue when exposed to iodine (I+). The is brownish green, about 10–15 μm high, and turns red-violet when treated with
nitric acid (N+). (filament-like structures in the hymenium) may occasionally branch and fuse together. They are roughly 2 μm wide, with their tips thickening to 3–4 μm. The
asci, which are the spore-bearing cells, typically contain 8 spores each and measure 50–65 by 11–13 μm. The themselves are simple,
ellipsoidal, colourless, and measure 10–17 by 5–8 μm. (asexual reproductive structures) are immersed in the thallus, with black, (point-like)
ostioles. The (asexual spores) are (rod-shaped) and measure 9–12 by 1 μm.
Lecidea lygommella does not produce any secondary chemicals. Its lookalike
Lecidea lygomma, in contrast, contains
norstictic acid as a major
lichen product and
connorstictic acid as a minor product. ==Habitat and distribution==