It differs from related species by its densely spiny, tomentose leaf sheaths and distinctive, yellow cross veins.

Type:— VIETNAM. Khanh Hoa: road 653B from Nha Trang to Da Lat, 54 km from Nha Trang, 67 km before Da Lat, near border with Lam Dong province, Hon Giao mountain, 12.180N, 108.733E, undisturbed forest on steep slope, 1000 m elevation, 21 April 2013, A. Henderson & Nguyen Quoc Dung 3869 (holotype, VFM! isotypes, AAU!, NY!).

Stems clustered, to 8 m tall, 1–1.3 cm diameter with sheaths, ca. 0.6 cm without sheaths. Sheaths densely brown tomentose initially, green, densely covered with greenish-yellow spines to 1.1 cm long, these tomentose as the sheaths; ocreas short, membranous, early deciduous; knees present; flagella 190 cm long; petioles 8–22 cm long, spiny along the margins and abaxially; rachises 38–60 cm long with 7–10, oblanceolate pinnae per side, these arranged in remote clusters, leathery, with prominent, yellow cross-veins, the basal pair oriented towards the sheath (especially on more apical leaves), distalmost few (usually four) in a cluster with the distalmost pair oriented away from the sheath and the adjacent pair oriented at a 45º angle to the rachis, the apical pair joined for 7.5–8.5 cm, middle pinnae 15–23.5 cm long, 3.4–4.3 cm wide at widest point, minutely spiny along the margins, not spiny on the veins. Staminate inflorescences 300 cm long, flagellate; rachis bracts tubular, terminating well below the partial inflorescences; partial inflorescences branched to 2 orders; rachillae 2.5–4.5 cm long; staminate flowers not seen; pistillate inflorescences similar to staminate but branched to 1 order; rachillae 2.5–2.7 cm long; dyad bracteoles obscure, campanulate, one side of mouth acuminate, visibly ribbed, the base free from the preceding neuter flower and without an impression of its bracteole; pistillate flowers 3.5 mm long before anthesis; calyx 3.5 mm long, tubular, lobed at the apices; corolla 3 mm long, tubular, with 3, valvate lobes at the apices; fruits (immature) globose to ellipsoid, 0.9 cm long, 0.7 cm diameter, 1-seeded; endosperm homogeneous or scarcely ruminate; embryo basal.

Distribution and habitat:— Endemic to southern Vietnam in Khanh Hoa Province in montane rain forest on steep slopes at 900–1000 m elevation.

Local names and uses:— may rac. No uses reported.

Additional specimen examined:— VIETNAM. Khanh Hoa: Khanh Vinh district, road from Khanh Hoa to Da Lat, 12.200N, 108.733E, 900 m, 12 July 2010, A. Henderson et al. 3711 (IEBR, NY).

Discussion:— In Evans et al. ’s (2001a) treatment of Calamus in neighboring Laos, this palm would key to a group of three species: C. oligostachys, C. solitarius, and C. tetradactylus, based on the presence of a flagellum and strongly grouped pinnae lying in one plane. Calamus flavinervis differs from all of these in its pinnae with yellow cross-veins. In the same study of Calamus cited above (Henderson, in prep.), Calamus flavinervis is placed in the same clade of 11 species as C. batoensis. It differs from all of these by its densely spiny, tomentose leaf sheaths and pinnae with distinctive yellow cross-veins.