EXACT systematic work is essential for oceano-graphical plankton studies, since we must know the species with which we are dealing before we can use them in any practical way. Mr. Hendey's extensive monograph on the diatoms of the Southern Ocean is most useful and is a careful study entailing a large amount of labour. With so much material it has been possible to review the inside variation present in most groups and in the individual species. The interesting fact is clearly brought out, agreeing with Wimpenny (1936) that the diatom frustules in cold water are smaller, thicker-walled and more vigorous than those from warmer waters, the reverse of what is known to exist in many marine animals. The holoplanktonic flora of the cold water from the oceanic habitats forms the major part of the collection and consists of a small number of discoid and a large number of filamentous pennate forms. The author, however, in his classification discards the usual division into Gentries and Pennatee, and his method of regarding many so-called species and varieties as "phases" of one species reduces considerably the number of species recognized. Thus, to take one example, Corethron according to Hendley is a monotypic genus, Corethron criophilium, much discussed by many workers, being the only species with many phases. This at least tends to simplify matters, and a reduction of the ever-increasing number of species is all to the good.