SUMMARYThe vegetative, floral and fruit morphology and anatomy ofChlaenosciadium gardneri,a monotypic, hydrocotyloid genus of Umbelliferae from Australia, is described in some detail. On the basis of comparisons of these observations with available reports for other genera of the tribe, as well as examination of herbarium material of these genera, it is apparent thatChlaenosciadiumis a distinct taxon which exhibits affinities withCentellaof Drude's Hydrocotylinae andXanthosia, PentapeltisandSchloenolaenaof his Xanthosiinae. The characters which lead to such a conclusion include: the leaf morphology and over‐all aspect, the paracytic stomata; the nature of the trichomes; the branching pattern of the inflorescence; the type of bracts and bractlets of the inflorescence; the type of petals; the anatomy of the fruit; and in particular, the presence of transverse vascular bundles in the flower and fruit. The absence of sepals is the only feature which readily excludesCMae. no‐sciandium,and evenCentella,from the Xanothosiinae. In the light of this investigation there is a need for a re‐evaluation of relationships within the subtribes of the Hydrocotyleae as defined by