INTRODUCTION In 1956 Kinne (1956, p. 257) published the description of a new athecate hydroid, Perigonimus megas, discovered by him in the Nordostseekanal, a canal connecting the North Sea with the Baltic. Kinne's new species resembles a well known and widely distributed fresh- and brackish-water hydroid, Cordylophora caspia (Pallas), in fact the resemblance between both species, though belonging to two different families of athecate hydroids (Perigonimus megas to the Bougainvilliidae; Cordylophora caspia to the Clavidae) is such that confusion is likely to occur and Kinne pointed out two occasions where the species have been confused by students of hydroids from the Netherlands. Both cases will be discussed below. The fact that on one of these occasions I have myself confused both species and contributed to the confusions arising from the misidentification, has prompted me to go through all available material of both species in the collections of the Zoological Museum at Amsterdam (Z.M.A.), the collections of the Division for Delta1) research of the Hydrobiological Institute, (= Delta Institute), Yerseke (D.I.), and the collections of the Rijksmuseum van Natuurlijke Historie, Leiden (R.M.N.H.). These studies brought forward the surprising result that Kinne's Perigonimus megas was a very common species in the former Zuiderzee, where it has now completely disappeared. The study of the very abundant material from the Netherlands has shown that both species, in well preserved form and especially when bearing gonophores, can be separated without any difficulty. The well