Hydromedusae with considerable variation in form, with umbrella usually deep bell-shaped; with gonads almost invariably situated on stomach, very rarely extending perradially on subumbrella; with or without ocelli; without statocysts. Hydroids always athecate.

Hydromedusae with umbrella usually hemispherical or flattened; with gonads on radial canals; marginal sense organs, when present, in form of cordyli or marginal vesicles of ectodermal origin; occasionally with ocelli.

Hydromedusae with umbrella margin entire and not divided into lobes; with thickened marginal nematocyst ring; with radial canals; with gonads usually confined to radial canals; with solid marginal tentacles, or with both solid and hollow tentacles, situated on the margin of the umbrella; with sensory clubs with endodermal axis which may be free or enclosed.

Trachyline medusae whose body is divisible into aboral and oral halves, separated by a line of tissue lying in the bottom of a constriction, this tissue representing the margin of the umbrella which is divided into four flaps acting as swimming lappets; the subumbrellar part of the body is highly reduced and consists of only the manubrium and the tissues on the adoral surfaces of the lappets. Without tentacles, but with statocysts on the oral sides of the lappets; statoliths endodermal. Gonads of ectodermal origin; development direct.

Scyphomedusae with the umbrella margin cleft into lappets; with a single mouth opening provided with simple lips; with sense organs (rhopalia) and solid marginal tentacles arising from clefts between the lappets; with a circular, coronal furrow in the exumbrella, and peripheral to this a zone of gelatinous thickenings (pedalia) divided by radiating clefts alternating in position with the tentacles and rhopalia.

Scyphomedusae with umbrella margin cleft into lappets; without a coronal furrow and without pedalia; with (or without) hollow marginal tentacles and with marginal rhopalia; with a single central mouth opening provided with four large curtain-like or gelatinous lips; with gonads in sac-like folds of the endodermal wall of the subumbrella.

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.