An expanded circumscription and revision of the Western Australian genus Balaustion (Myrtaceae: Chamelaucieae: Hysterobaeckeinae)
Balaustion Hook. s. lat. comprises 18 species that are characterised by having a large cavity on the inner surface of their seeds. Other characters found in all or most of the species are a low growth form, sub-epidermal tissue on the young stems that disintegrates into fibres, one-flowered peduncles, scarcely keeled sepals, and very large, sessile placentas. The type species of Balaustion has a distinctive anther type and particularly large, colourful flowers adapted to bird-pollination, but white is the most common petal colour in the genus. New combinations are made for three species previously included in Baeckea L. s. lat.: Balaustion exsertum (S.Moore) Rye, B. grandibracteatum (E.Pritz.) Rye and B. grande (E.Pritz.) Rye. A neotype is selected for Baeckea grandibracteata E.Pritz. and lectotypes chosen for B. grandis E.Pritz. and the type species Balaustion pulcherrimum Hook. The new species and subspecies are B. baiocalyx Rye, B. bimucronatum Rye, B. filifolium Rye, B. grandibracteatum subsp. meridionale Rye, B. grandibracteatum subsp. juncturum Rye, B. hemisphaericum Rye, B. interruptum Rye, B. karroun Rye, B. mukinbudin Rye, B. multicaule Rye, B. polyandrum Rye, B. quinquelobum Rye, B. spenceri Rye, B. tangerinum Rye, B. thamnoides Rye and B. unguiculatum Rye. Most of these taxa are conservation-listed in Western Australia. Three sections are recognised: sect. Balaustion is monotypic, sect. Nonfibrosa Rye comprises three species with few ovules, and sect. Tilophloia Rye comprises 14 species characterised by stem tissue that tends to disintegrate into numerous fibres. One possible case of hybridisation between two of these sections is discussed.