The affinities, structure and distribution of species of the genus Tetraplatia Busch, 1851 have been discussed recently by Hand (1955), Beyer (1955) and Rees & White (1957 a), but ever since these curious bipyramidal medusae (without a typical medusan bell, marginal tentacles, or gastrovascular canals) were described by Busch they have provided much interest for the systematise There is unanimous agreement that the clearly discernible nematocyst tracts, and the body wall of ectoderm and endoderm separated by a thick, well-defined mesogloea, place them within the Cnidaria as known at present. Tetraplatia is usually regarded as the only member of the hydrozoan order Pteromedusae, but Komai (1939) strongly advocates scyphozoan rather than hydrozoan affinities. The purpose of this paper, however, is not to discuss the broad relationships of the Pteromedusae, but to give further evidence that Tetraplatia chuni Carlgren, 1909 is a good species. (The structure and affinities of Tetraplatia will be discussed in another paper.)