A new family is proposed for Hamondia superba gen. et sp. nov., a shield-shaped harpacticoid collected from washings of sponges from Port Phillip, Australia. It is concluded that the closest relatives of the Hamondiidae fam. nov. currently belong to the heterogeneous thalestrid genus Rhynchothalestris Sars, 1905 and for that reason the latter is revised. Rhynchothalestris rufocincta (Brady, 1880) is redescribed and designated as type species for Ambunguipes gen. nov., comprising also the Indo-Pacific A. Similis (A. Scott, 1909) which is re-instated. Ambunguipes vanhoeffeni (Brady, 1910) is ranked as species inquirenda within the genus. Because of its body ornamentation R. Cornuta Geddes, 1969 is transferred to Lucayostratiotes gen. nov. The Ambunguipedidae fam. nov., accommodating Ambunguipes and Lucayostratiotes are regarded as the sister-group of the Hamondiidae on the basis of the loss of the seta on the first antennular segment, the presence of bifid spinules on the antennary exopod, the presence of a modified seta on the maxillary precoxal endite, the sexual dimorphism of leg 3, the detailed structure of the female genital complex, and the asymmetry of the male P6. The diagnosis of the subfamily Rhynchothalestrinae, encompassing the genera Rhynchothalestris and Peltthestris Monard, 1924 is amended, and a redescription of the former's type species R. Helgolandica (Claus, 1863) is presented. The genus also contains R. Tenuis Chislenko, 1971 and R. Campbelliensis Lang, 1934 grad. nov. which is elevated to full species rank. The inadequately described R. Tenuicornis (Brady, 1910) is ranked as species inquirenda. The monotypic genus Peltthestris is regarded as a valid genus. It is suggested that the Rhynchothalestrinae represent an early offshoot in the evolution of the Thalestridae and that the Thalestrinae and the Dactylopusiinae are more closely related to each other than to any other subfamily. Rhynchothalestris agigensis Serban, 1959 has no close relationship with either the Rhynchothalestrinae or the Ambunguipedidae and is tentatively considered incertae sedis within the Dactylopusiinae. The discovery of H. Superba and the redescription of A. Rufocincta have raised the maximum number of antennular segments in male harpacticoids to 14. The terms `epicopulatory bulb' and `epicopulatory plate' are coined for peculiar structures of the female genital complex. The development of `setoid elements' versus addition of novel setae is briefly mentioned.