Thallus stratified, not umbilicate
1a Lobes hollow, partially hollow, or solid but distinctly appearing inflated
2a Lobes solid but swollen and appearing inflated; generally on rock, occasionally on moss over rock; medulla P-
(uncommon forms of Hypogymnia imshaugii with solid lobes will key here)
2b Lobes hollow; substrate and spot tests various
3a Upper surface perforate; thallus closely flattened to the substrate; w Cas
3b Upper surface not perforate, but lobe tips often perforate; thallus appressed or loosely attached to the substrate; widespread
1b Lobes neither hollow nor appearing inflated
4a Lower surface with distinct or diffuse veins
4b Lower surface not veined, though often variously textured (smooth, wrinkled, warty, etc.)
5a Lower surface mottled with pale to brownish tomentum and irregular bare patches; thallus usually large
5b Lower surface variously patterned or smooth but not mottled as above; thallus size various
6a Thallus yellow-green to greenish (containing usnic acid) when dry, sometimes darkening in exposed habitats
With faint yellowish tint (usnic acid)
6b Thallus white, gray, greenish gray, brown, or blackish (usnic acid lacking or not apparent)
7a Lobes narrow, < 1.5 mm wide; thallus small, generally < 3 cm broad (Note: occasional narrow-lobed forms can be found for most medium- to broad-lobed species; these should be keyed in Group 4)
8a Upper cortex K+Y or K+ dirty Y, light colored; upper surface mostly white to gray; lower surface generally white, occasionally brown or black
Narrow-lobed foliose, pale above, cortex K+Y, usnic acid lacking
8b Upper cortex K- or cortex too dark to see the K reaction; upper and lower surfaces various
Narrow-lobed foliose; gray (or whitish) pruinose, brown or black; cortex K-
7b Lobes generally broader, generally > 1.5 mm wide; thallus medium to large, often > 3 cm
Medium- to broad-lobed foliose; not yellowish (usnic acid lacking)