Hypocalymma minus (Strid & Keighery) Keighery

Most members of the south-western Australian genus Hypocalymma (Endl.) Endl. have seeds with a shallowly to very deeply pitted testa. Two small groups of species, both revised here, differ in having a smooth or colliculate testa. Both groups occur in humid environments in the extreme south-west of Western Australia and show vegetative adaptations consistent with this habitat preference. The larger group, Hypocalymma sect. Cardiomyrtus Schauer, is reinstated. A new combination, H. minus (Strid & Keighery) Keighery, is made, bringing the total species recognised to five, and lectotypes are selected for two synonyms, H. boroniaceum F.Muell. ex Benth. and H. hypericifolium Benth. The other group is extremely rare, consisting of the possibly extinct species H. connatum Strid & Keighery, and a new species known from a single population, H. verticillare Rye. The latter group, named here as Hypocalymma sect. Verticilla Rye, is readily disitinguished by its 3-ridged young stems and consistently whorled leaves, and the new species is unusual in having extreme differences in the sizes of the anthers on different stamens within each flower. Molecular data indicate that sect. Verticilla is sister to sect. Cardiomyrtus. Evidence of taxonomic relationships is also presented from studies of hybridisation and insect associations.