This work was started many years ago in the form of a card-index for my own use. At the beginning the building up of the index involved a great deal of work, but later on it saved me time for my studies, and it was not difficult to keep it up to date. One day my friends, Dr F. S. Russell, director of the Marine Biological Laboratory, Plymouth, and Dr W. J. Rees, British Museum (Nat. Hist.), London, saw this index, and they repeatedly urged me to put it into a form suitable for publication for the benefit of other students of medusae. I agreed, though hesitatingly, because my time was much occupied. A grant from the Browne Research Fund of the Royal Society of London, however, enabled me to employ an assistant, Mr J. Liitzen, who under my supervision could carry out the necessary revision and the typing, as well as some translation from Danish or other languages into English; he has done it remarkably well, and I wish to thank him for his industrious and painstaking collaboration.