Description from Flora of China
Shrubs or trees deciduous. Leaves alternate, petiolate, margin doubly serrate or lobulate. Male inflorescence pendulous, subsessile, cylindric, spicate-cymose, naked during winter, with numerous overlapping bracts; bracts each usually subtending 2 bracteoles and 1 male flower; stamens 2-8, inserted at middle part of bracts; anthers with 2 separate thecae, often pubescent at apex. Female inflorescence a compact cluster of several flowers; bracts campanulate or forming a tubular sheath, apex usually lobed; flowers paired, enclosed by a bract; calyx adnate to ovary, 4-8-lobed at apex; ovary with 1 or rarely 2 ovules in each cell; style 2-cleft to base. Fruit a subglobose or ovoid nut, enclosed or surrounded by enlarged bracts.
About 20 species: China, Japan, Korea, Mongolia, Russia; Europe, North America; seven species (four endemic) in China.
Corylus formosana Hayata (Gen. Ind. Pl. Form. 72. 1916) was described from Taiwan but no specimen is extant. It was treated as an uncertain species by J. C. Liao in Fl. Taiwan, ed. 2.