Diminutive tufted to mat-forming obligate rheophytes to 5 cm tall. Stem somewhat elongated, eventually sub erect to decumbent and rooting, 1–4 cm long, 1.5–2 mm in diam., more or less clothed in conspicuous fibrous-netted cataphyll, ligule, and leaf base remnants, the older parts becoming bare. Leaves few to numerous together; petiole 0.7–1 cm long, ca. 0.4 mm diam., adaxially canaliculate, sheathing at the extreme base; petiolar sheath with wings extended into a very narrowly triangular ligular portion 1 cm long drying dark red-brown; blade thinly coriaceous, adaxially dark green, paler abaxially, very narrowly linear, 1–2.5 cm long 3 2–2.5 mm wide, base narrow cuneate, apex mucronate to cuspidate, apiculate for 0.5–0.8 mm, margin somewhat thickened and slightly revolute; midrib abaxially very prominent, adaxially prominent, primary lateral veins indistinguishable from the interprimary venation, diverging at 20–35° and running to a more or less thick marginal vein; secondary venation adaxially and abaxially very faint to completely obscure; tertiary venation mostly completely obscure in living material, forming a faint tessellate reticulum in dry material. Inflorescence solitary; peduncle somewhat stout, exceeding the petioles, ca. 8 mm long x 0.4 mm diam., terete, pale green; spathe more or less ovoid, not constricted, ca. 1.5–1.8 cm long and apically beaked for 3–4 mm; lower part funnel shaped, green, persistent, the upper part gaping, glistening white, caducous, apical beak medium green. Spadix subcylindric to bluntly spindle-shaped, 6–7 mm long, ca. 2 mm diam.; pistillate flower zone ca. 1 mm long, reduced to a single whorl of pistils, ca. 2 mm diam.; pistils crowded, subglobose, ca. 0.5–1.5 mm diam.; stigma sessile, discoid, centrally impressed, about the same width as the ovary; interpistillar staminodes confined to a row along the spathe/spadix adnation, shortly stipitate, broadly to narrowly spindle shaped to almost filamentous, about the height of the pistils; sterile interstice defined a partial to complete whorl of staminodes, these globular, 0.5–0.6 mm in diam.; staminate flower zone ca. 5–9 mm long, comprised of 1–2 rows of fertile flowers; staminate flowers pubescent, crowded, each comprised of two stamens arranged in longitudinally aligned pairs, truncate, deeply excavated with the thecae together on the inner (with respect to the stamen pairs) side of the anther, ellipsoid to ellipsoid-oblong from above, 0.8–1 mm long x 0.4–0.6 mm wide; thecae separated by a ridge forming a septum in the cavity, very shortly horned, with the horns inside the lip of the anther cavity; appendix ca. 1–4 mm long, rounded; appendix staminodes more or less irregularly globular to ellipsoid, mostly flat topped, occasionally more or less excavated, ca. 0.5–0.8 mm diam. Fruiting material not seen.
Aridarum orestum shares the occurrence of pubescent staminate flowers with A. velutandrum, but is readily differentiated in having elongated stems clothed in conspicuous fibrous-netted cataphyll, ligule, and leaf base remnants (vs. stems very short and naked), and by the softly leathery (not very stiff), almost flat (vs. sharply Vshaped), leaf blades with a tubular (not sharply pointed) leaf tip.
Aridarum orestum known only from the Type locality. The Type material is plentiful, with each sheet carrying several quite large clumps, indicating that the plant is (or at least was) plentiful.
Aridarum orestum occurs as a rheophyte on sandstone riverside boulders pockets in mossy upper hill forest at ca. 800 m asl.