The microsoroid ferns are one of most diverse lineages in the family Polypodiaceae (Kreier et al. 2008). Within microsoroid ferns, the genus Leptochilus Kaulfuss (1827: 147) sensu Nooteboom (1997) was estimated to contain 20–30 by Zhang & Nooteboom (2013), but there has not been a modern revision of the genus published. Species of the newly defined Leptochilus with Kontumia S.K.Wu & K.L.Phan (2005: 245) included (Kim et al. 2013) are morphologically variable. Their lamina can be simple, partially lobed to deeply 1-pinnatifid, and exceptionally 3-pinnatifid (in Kontumia; Wu et al. 2005). In addition, the habit, dimorphism, shape of sori are also diverse in the genus.
A large number of endemic ferns which have been partially described in the recent studies occur in the limestone areas of southern eastern Asia (e.g., He & Zhang 2010, Lu et al. 2014). In our fieldwork in the limestone areas of northern Vietnam in 2013, we collected some samples of Leptochilus. These samples have regularly 1-pinnatifid lamina, similar to L. ellipticus and the related species, but the sori of the these samples are oblong and close to the leaf margins, different from all other species in the genus. We describe it here as Leptochilus oblongus.
Leptochilus oblongus Li Bing Zhang, Liang Zhang & Ngan T. Lu, sp. nov.
Type:—VIETNAM. Hoa Binh Province: Lac Son District, Ngoc Son Ngo Luong Nature Reserve, elev. 640–700 m, 20°25’N/105°20’e, on limestone rocks in disturbed rainforest, 20 November 2013, L.-B. Zhang, L. Zhang & N. T. Lu 6299 (holotype VNMN!; isotypes CDBI!, MO!).