It differs from related species by its rachis bracts which are swollen at the apices and recurved pistillate rachillae which have the dyads arranged in alternate rows but not opposite so one side of the rachillae is without flowers.

Type:— VIETNAM. Quang Ngai: Ba To District, road from Ba To to Ba Cung, steep slope in forest near road, 14°44’N, 108°53’E, ca. 600 m, 24 April 2013, A. Henderson & Nguyen Quoc Dung 3881 (holotype, VFM! isotype, NY!).

Stems clustered, 1–10 m long, 0.5–0.8 cm diameter with sheaths, ca. 0.6 cm without sheaths. Sheaths greenish-white, longitudinally striate, not tomentose, with scattered, brown-tipped spines to 0.2 cm long; ocreas short, membranous, early deciduous; knees present; flagella 72 cm long; petioles 2.5–11 cm long, spiny along the margins and abaxially; rachises 12.5–30 cm long with 3–4, elliptic pinnae per side, these arranged in remote clusters, the proximal pair oriented towards the sheath, the 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 8.5–12.7 cm, middle pinnae 15–26 cm long, 3.7–4.7 cm wide at widest point, minutely spiny along the margins and with a few spines at the apices, not spiny on the veins. Staminate inflorescences 76 cm long, flagellate, the first partial inflorescence subtended by the prophyll; prophyll and rachis bracts tubular, swollen at the apices, just covering the bases of the partial inflorescences; partial inflorescences branched to 2 orders; rachillae 0.5–1 cm long; staminate flowers not seen; pistillate inflorescences 31–62 cm long, similar to staminate but branched to 1 order; rachillae 1.5–2 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; dyads arranged in alternate rows but not opposite, one side of the rachillae without flowers, the rachillae recurved; pistillate flowers 6 mm long (post anthesis); calyx 2.5 mm long, tubular, lobed at the apices; corolla 4 mm long, tubular, with 3, valvate lobes at the apices; fruits (immature) globose, 1.1 cm long, 0.9 cm diameter, 1-seeded; endosperm homogeneous or scarcely ruminate; embryo basal.

Distribution and habitat:— Endemic to central Vietnam in Quang Ngai Province in secondary, evergreen forest at 600 m elevation.

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

Additional specimens examined:— VIETNAM. Quang Ngai: Ba To District, road from Ba To to Ba Cung, steep slope in forest near road, 14°44’N, 108°53’E, ca. 600 m, 18 July 2012, Henderson et al. 3808 (NY, VFM); same locality, same date, Henderson et al. 3809 (NY, VFM); same locality, 14 September 2011, Nguyen Quoc Dung 2036 (NY, VFM).

Discussion:— In Evans et al. ’s (2001a) treatment of Calamus in neighboring Laos, C. batoensis (named for the district in which it occurs) would key to a group of three species: C. oligostachys Evans et. al. (2001b: 242), C. solitarius Evans et al. (2000: 932), and C. tetradactylus Hance (1875: 289), based on the presence of a flagellum and strongly grouped pinnae lying in one plane. Calamus batoensis differs from all of these in its swollen rachis bracts and recurved pistillate rachillae. In the course of a revision of Calamus (Henderson, in prep.), phylogenetic analysis of morphological data comprising 110 characters taken from 3133 specimens representing 316 species of Calamus, five of Ceratolobus Blume ex Schultes & Schultes (1830: lxxx), five of Daemonorops Blume in Schultes & Schultes (1830: 1333), one of Retispatha Dransfield (1980a: 529), and three of Pogonotium Dransfield (1980b: 763), with Plectocomia Martius & Blume in Schultes & Schultes (1830: 1333) as outgroup, shows that Calamus batoensis is placed in a clade of 11 species, all from Laos and Vietnam, characterized by the short, membranous ocrea. These are: C. oligostachys, C. solitarius, C. tetradactylus, C. parvulus Henderson & Nguyen Quoc Dung (2010: 30), C. crispus Henderson et al. (2008: 191), C. dioicus Loureiro (1790: 211), C. kontumensis Henderson et al. (2008: 193), C. bimaniferus Evans et al. (2000: 936), and two new species described below (C. flavinervis, C. quangngaiensis). Calamus batoensis differs from all of these by its rachis bracts which are swollen at the apices and recurved pistillate rachillae which have the dyads arranged in alternate rows but not opposite so one side of the rachillae is without flowers. Although these two character states occur in unrelated species of Calamus, they only occur in this clade in C. batoensis.