Plagiochasma eximium is a rather variable species, but can be distinguished by its robust, yellow-green thalli with large, deep pink to wine-red ventral scales narrowed above into (1)2 or 3 tapering appendages which are only a little constricted or pleated at the base. Ventrally the flanks are wrinkled and deep red, not dark purple. Elaters from the few spore-bearing plants studied are bispiral.
Thalli robust, in gregarious patches; yellow-green, finely and irregularly areolate, air pores faintly visible, margins narrowly to widely dark red, scalloped and undulate; when dry, margins tightly inflexed. Branches simple or once pseudodichotomously furcate, sometimes articulate with one or more apical innovations, 16.5-22.0 mm long, 5.0-8.5 mm wide, ± 750 µm thick medianly, in section 7-11 times wider than thick, apex notched, with lanceolate, reddish pink or dark red scale appendages recurved over edge; groove absent, thallus margins acute, thin; flanks sloping obliquely; ventral face medianly keeled, green. Scales in 2 ventral rows, asymmetric, obtusely triangular, deep pink to wine red, body ± 950 µm long, margins occasionally with projecting papillae, base arched, ± 1650 µm wide, narrowed above and slightly constricted or pleated where joined with 1 or 2, sometimes 3 tapering appendages, 650-900 µm long, basally 250-350(-400) µm wide, apex not acuminate but often unicellular, ± 14 smaller oil cells scattered throughout scale. Dorsal epidermal cells unistratose, hyaline, polygonal, 25-40 x 20-35 µm, walls with trigones at corners, smooth externally; air pores slightly raised, simple, 12.5-27.5 µm wide, surrounded by innermost ring of small, collapsed cells, then by 3 concentric rings of 7 or 8 larger cells in each, radial walls sometimes thickened. Assimilation tissue 375-475 µm thick, air chambers empty, in several storeys, bounding walls chlorophyllose; storage tissue well developed; oil cells scattered throughout tissues. Monoicous, but male and female receptacles sometimes on separate plants. Androecia medianly at base of smaller, articulate, apical innovation of leading branch, sessile cushions, kidney- or sausage-shaped, basally encircled by apically tapered, dark red paleae, 250-500 x (90-) 100-120 µm, margins sometimes with projecting papillae. Gynoecial receptacles medianly near apex of branch, single or occasionally paired when branches bifurcate, enclosed by arching, hyaline, tapering paleae, ± 550 x 80 µm, margins sometimes with papillae; carpocephala 3.25 x 3.25 mm, when 4 lobes present. Stalk 3-6(-9) mm long, ± 850 µm wide, striate and dark red. Involucre bilabiate. Spores 82.5-90.0 µm diam., triangular-globular, polar, brown, translucent; wing up to 12.5 µm wide, porate at corners, margins undulate, finely crenulate; ornamentation similar on two faces; distal face with 4 or 5 alveoli across, 22.5-32.5 µm wide, walls wide and finely granular; proximal face with distinct tri-radiate mark, arms ± 5 µm high, mostly 4 shallow alveoli on each of 3 facets, walls sparsely sprinkled with granules. Elaters light brown, 175-225 µm long, 15-20 µm wide in middle, tapering toward ends, 5.0-7.5 µm wide, laxly bispiral or spirals interrupted.