Cyphostemma calcarium is most closely comparable to C. rutilans Desc. in morphology. It differs from the latter in having distinct pubescent branches (vs. branches entirely glabrous in C. rutilans); leaves minutely puberulous and shiny on the adaxial surface, abaxial surface densely whitish pubescent to velvety particularly on the veins (vs. leaves entirely glabrous and shiny on both sides in C. rutilans); and leaflets broadly oblong or elliptic, base cuneate (vs. leaflets narrowly ovate, base subcordate in C. rutilans). Seeds of C. calcarium are ellipsoid in outline, 7–7.5 × 3–3.5 mm, surface rugose to ± muricate (vs. seeds globose, 5–6.5 mm in diameter, surface strongly rugose in C. rutilans).

Madagascar. Antsiranana: Diana, Ankarana Special Reserve, Tsingy Rary, 12°56'24.00"S, 49°07'04"E, 97 m, 16 May 2013, Rakotovao C. et al. 6376 (holotype: TAN!).

Succulent erect shrub, up to 2 m tall. Old stems swollen, succulent; bark smooth, lenticellate; branches brown to reddish, shortly pubescent. Tendrils absent. Stipules triangular to ± falcate, 4–5 × 1.5–2.5 mm, soon caducous. Leaves 3-foliolate, central leaflet often drooping, somewhat thick and fleshy when fresh, becoming coriaceous when dry, usually folded upwards along the midrib; leaflets 3–5 × 1.5–2.5 cm, broadly oblong or elliptic, base cuneate, rounded to obtuse at the apex, margin shallowly denticulate; minutely puberulous and shiny on the adaxial surface, abaxial surface densely whitish pubescent to velvety particularly on the veins; venation closely reticulate, prominent. Petioles 1.5–2.5 cm long. Petiolules equal, up to 1 cm long. Inflorescence a compound dichasium, terminal, ca. 7.5 cm long, very shortly pubescent; bracts inconspicuous; pedicels 2–4 mm. Floral buds ± 2.5 mm long, minutely puberulous or glabrescent; sepals ± 0.5 mm long; petals reddish; stamens 4, filaments cylindrical, ca. 2.2 mm long, anthers ca. 0.8 mm long; ovary glabrous, styles ± 2.5 mm long. Fruits ellipsoid, 9–12 × 5–7 mm, glabrous. Seeds broadly ellipsoid, 7–7.5 × 3–3.5 mm, rugose; base rostrate; beak conspicuous; apex revolute; rugae apex shallowly conspicuous on both surfaces; chalaza linear, sinuate, up to 6 mm long (ca. 6/7 of seed length); ventral ridge raised, elongate but widened in the middle, extending up to 6/7 of seed length; endosperm m-shaped in cross-section.

Flowering and fruiting around May.

The epithet of the species refers to the habitats on limestone outcrops.

It grows on limestone outcrops in northern Madagascar at an altitude of 90–300 m. (Fig. 1)

The new species is endemic to Madagascar with distribution restricted to its type locality. It is assessed here as Critically Endangered (CR) according to the IUCN Categories and Criteria (IUCN 2019). Even though the species occurs within a protected area, succulent plants are still highly sought after by collectors for their horticultural values. Seeds of Cyphostemma calcarium should therefore be collected, banked, and propagated to ensure its long-term conservation.

This species is described from materials collected by Rakotovao C. et al. in 2013. It was initially identified as Cissus pileata Desc., but it clearly belongs to Cyphostemma in having constricted flower buds and floral disks with four free glands. These characters, together with its M-shaped endosperm as viewed in cross sections of the seeds and the presence of extra layers of endotestal sclereids covering the ventral infolds in cross-section, clearly distinguish the new species from Cissus L. A summary of some diagnostic characters that differentiate this new species from other shrubby species of Cyphostemma in Ankarana Special Reserve is provided in Table 1.