When fertile, T. hypophylla is recognized by the ventrally displaced capsule contained in a shiny black pouch at the apex of the thallus; sterile plants are distinguished by their dark green, somewhat leathery appearance, the conspicuous white-encircled air pores and reddish to purple-black, obliquely triangular ventral scales with a single appendage.
Thalli medium-sized, in crowded patches; dark green, rather leathery, faintly areolate, air pores large, white-encircled, margins purple and entire to somewhat crenate; when dry, margins incurved, thalli almost tubular. Branches simple to repeatedly furcate or with ventrolateral or apical innovations, linear to ligulate, up to 30 mm long, 2.7-3.5 mm wide, 550 µm thick medianly, in section 5 or 6 times wider than thick, apex slightly notched or entire; groove absent, thallus margins acute, thin; flanks sloping obliquely; ventral face medianly keeled, mostly deep purple. Scales in 2 ventral rows, 875-1750 µm long, up to 1325 µm wide across base, tips not, or hardly reaching thallus margins, obliquely triangular, imbricate, reddish to dark purple, margins entire or sometimes with papillae or fimbria, apically continuing into long-acuminate appendage ± 375 µm long, oil cells few, scattered. Dorsal epidermal cells unistratose, hyaline, rounded to oval, 22.5-37.5 x 17.5-25.0 µm, with conspicuous trigones; air pores somewhat raised, simple, oval, 52.5-67.5 x 35.0-40.0 µm, surrounded by 3 concentric rings of differentiated cells: innermost ring with ± 6 small, thin-walled, collapsed cells, outer 2 rows with 8-11 and ± 14 cells in each respectively, slightly thicker-walled. Assimilation tissue ± 80 µm thick, with air chambers in one storey, containing simple or branched, 2- or 3-celled chlorophyllose filaments, bounding walls hyaline; storage tissue ± 425 µm thick, cells closely packed; oil cells scattered throughout tissues. Autoicous or dioicous. Androecia terminal on short, ventrolateral branches, sessile discs, encircled above by low, frilly membrane; antheridia embedded, opening into conical protuberances. Gynoecia ventrally displaced below apex of thallus. Involucres enclosing single capsule, large, shiny black, mussel-like, bivalved, wall 4-layered, opening along narrow, central, vertical fissure. Pseudoperianth lacking. Sporophyte subsessile with distinct foot and short seta; capsule globose, wall with thickening bands, dehiscence by irregular rupturing. Spores (47-)62-77(-95) µm diam., light brown to dark reddish brown, anisopolar; distal face convex, ornamented with network of ridges, enclosing 16 or 17 polygonal areas, 12.5-17.5 µm wide, all covered by fine reticulum, alveoli smaller on sides and crests of primary ridges, larger within enclosures; proximal face flattish, generally with very irregularly contorted, crowded, reticulated ridges, separated by narrow fissures. Elaters yellowish brown, not tapering but sometimes branched, bispiral, up to 290 x 12.5 µm.