Type:— CHINA. Yunnan: Qiubei County, Xindian Town, Dadong Village, in a limestone sinkhole, elev. 2000 m, 24°02 ′ 36 ″ N, 103°46 ′ 25 ″ E, 17 April 2015, Mengqi Han & Jinquan Huang HMQ094 (holotype IBK!, isotype CDBI!).

Diagnosis:— Polystichum crassirachis is most similar to some individuals of P. normale Ching ex P.S.Wang & Li Bing Zhang in Zhang & Wang (2012: 250), but the former has a pinnate lamina and acute teeth on pinna margin, while the latter often has a bipinnate lamina and square teeth on pinna margin.

Plants perennial, evergreen, 20–30 cm tall. Rhizomes ascending, 1.5–2.1 cm long, ca. 1.7 cm diam., with remnant bases of old stipes; roots dull brown when dried, 5–9 cm long, ca. 0.3 mm diam. Leaves in tufts, 14–20 per rhizome; petioles 10–18 cm long, 0.8–1.2 mm diam., basal portions densely covered with scales, scales lanceolate to ovate-lanceolate, 1.5–2.5 × 0.6–1.2 mm, papery, dull brown, almost entire, margins with lighter color, apex acuminate; distal petiole scales similar but narrower, 0.9–1.8 × 0.3–0.8 mm, membranous, margins irregularly erose or short-ciliate, apex long-acuminate or caudate. Laminae oblanceolate, 1-pinnate, 15–24 × 2–3 cm, apex acuminate, yellowish green when dried; rachises 0.6–0.9 mm diam., scales ovate, ovate-lanceolate, or long-lanceolate, dull brown, 0.8–1.3 mm long including tip, 0.4–0.6 mm wide at base, margins sparsely ciliate, apex long-caudate. Pinnae 22–32 pairs, deltoid, 0.8–1.4 × 0.3–0.4 cm, basalmost 3–5 pairs slightly smaller toward lamina base, nearly 2/3 as large as middle ones, 0.6–0.7 cm apart, middle pairs 0.6–0.7 cm apart, all pinnae papery, alternate, acroscopic margins and distal basiscopic margins shallowly toothed, basiscopic margins forming a nearly right angle with rachis, apex acute to obtuse, base cuneate and asymmetric with acroscopic sides being much broader, petioles 0.3–0.5 mm long, abaxially with microscales, microscales lanceolate to ovate-lanceolate, brown, ca. 0.2 mm long, 0.1 mm wide at base; adaxially glabrous; veins visible abaxially but somewhat obscure adaxially, lateral veins free, single or forked. Lower 3–7 pairs of pinnae sterile; sori terminal on lateral veins of fertile pinnae, (1–)3–7 on acroscopic side of fertile pinnae, 0–3(–4) on distal basiscopic side of fertile pinnae, ca. 1 mm diam., closer to pinna margins than to midrib (centers of sori 0.4–0.8 mm from pinna margins, 0.4–1.5 mm from midrib), centers 1.2–1.6 mm apart from one another; indusia not seen. Spores round in polar view and elliptic in equatorial view; perispore sculpture cristate and granulate.

Geographical distribution:— Polystichum crassirachis is only known from southeastern Yunnan, China. It is most likely endemic to the sinkhole where it was discovered.

Ecology:— Polystichum crassirachis grows inside a limestone sinkhole at an elevation of 2000 m with humid and twilight conditions.

IUCN Red List category:—CR-Critically Endangered: Only one population with about 100 plants was seen in the field.

Etymology:—The species epithet is based on the Latin prefix, crassi-, thick, and the Latin noun, rachis, rachis, referring to the thick rachis of the species.

Taxonomic notes:— Polystichum crassirachis resembles P. subacutidens Ching ex L.L.Xiang (1994: 261) in having shallowly toothed pinna margins, but the former often has reflexed pinnae, thicker rachises, and 22–32 pairs of pinnae, while the latter has non-reflexed pinnae, thinner rachises, and 40–110 pairs of pinnae. A preliminary molecular analysis showed that the new species is most closely related with P. membranifolium Li Bing Zhang, M.Q.Han & Yan Liu (here described below) from northeastern Yunnan. However, these two species can easily be distinguished from each other in leaf texture and pinna margins (see below).