The Genetic Relationships of Australasian Rissoids. Part 1. Descriptions of New Recent Genera and Species from New Zealand and Kermadec Islands. By A. W. B. Powell. [Read before the Auckland Institute, 24 November, 1925; received by Editor, 11th December, 1925; issued separately, 1st February, 1927.] Plates 26–28. Prior to Iredale's (1914-15) Commentary, the Australasian Rissoids, prejudiced by their small size and infinite variety of design, were, with a few exceptions, lumped in a conventional manner in the genus Rissoa, of world-wide range, comprising species of very diverse origin. Iredale, in the above-mentioned paper, proposed eight new genera to cover Australasian forms and utilized five previously described, of which only two were proposed for southern shells. His new genera were, however, quickly adopted by Australasian systematists, the classification being recognized as a decided advance. Finlay, in a valuable paper entitled “N.Z. Tertiary Rissoids” (1924, p. 483), proposed still another genus, but the majority of his new species were easily and naturally located by means of Iredale's new series of genera. In the present paper sixteen new species are described, ten of which fall into previously described genera. In order, however, correctly to express the remaining six species, it has been found necessary to propose five new genera. Some of these new genera are, however, represented in other parts of Australasia by species that have hitherto been ascribed to other groups. If a genus is represented by only one group of undoubted genetic affinity, it should be kept so and not forced to contain species only superficially similar. If several of these aberrant forms themselves resolve into a compact group, it is, in the opinion of the writer, sufficient justification to warrant their generic separation, even on shell characters, where morphological evidence is unavailable. Then these naturally-compiled genera based on evolutionary lines become of great importance to the geologist for the purpose of discussing geographical distribution and geological correlation. It may be argued that this system unnecessarily complicates the nomenclature and that there is a danger of creating almost as many genera as species, but, on the other hand, there is little doubt that only a very small percentage of the New Zealand Rissoid fauna is known, and that further collecting will swell the ranks of these at present poorly represented types. Including the additions recently made by Iredale and Finlay and those described in this paper, the N.Z. Rissoid fauna now numbers sixty-nine species, representing twenty genera. Probably all these species are restricted to New Zealand, many being extremely local. Most of the genera are represented in either south-eastern Australia, Tasmania, or Kermadec Islands, but are not found elsewhere, and show little connection with the northern hemisphere series. Rissoina