When studying the marine species of the genus Cladophora in the Netherlands, I had the disposal of the material of the National Herbarium at Leiden, the herbaria of the Universities of Amsterdam, Groningen and Utrecht and those of the “Zoölogisch Station” at Den Helder and the “Koninklijke Nederlandse Botanische Vereniging” at Leiden. It is a pleasure to me to express my best thanks to the directors of the Institutes mentioned above, for putting the material at my disposal, and to Miss Dr J. Th. Koster, who gave me much valuable help. Especially the material of the National Herbarium at Leiden was very interesting to me, since it contains the herbaria of Kützing, Hauck, and very many reliable exsiccata, so it was possible to compare some Dutch species with the original specimens.