The Cubomedusæ are of more than passing interest among jelly-fish, both because of their comparative rarity and because of the high degree of development attained by their nervous system. One fact alone suffices to attract at once the attention of the student of comparative morphology--that here among the lowly-organized Cœlenterates we find an animal with eyes composed of a cellular lens contained in a pigmented retinal cup, in its essentials analogous to the vertebrate structure. Perhaps this and other facts about the Cubomedusæ would be more generally known, had they not been to a certain extent hidden away in Claus’s paper on Charybdea marsupialis (’78), which, while a record of careful and accurate work, is in many respects written and illustrated so obscurely that it is very doubtful whether one could arrive at a clear understanding of its meaning who was not pretty well acquainted with Charybdea beforehand. Before Claus’s paper was received at this laboratory, H. V. Wilson went over essentially the same ground upon a species of Chiropsalmus taken at Beaufort, N. C. When the article on Charybdea marsupialis appeared, however, the results were so similar that Wilson did not complete for publication the careful notes and drawings he had made. Haeckel’s treatment of the Cubomedusæ in his “System” (’79) in the Challenger Report (’81) is much more lucid than Claus’s; but the extended scope of his work and the imperfect preservation of his material prevented a detailed investigation, and for a more complete and readily intelligible account of the structure of the Cubomedusæ a larger number of figures is desirable. In the foregoing facts lies whatever excuse is necessary for repeating in the present paper much that has already seen print in one form or another. PART I: SYSTEMATIC. It seems advisable first of all to establish the systematic position of the two newly found species, Charybdea Xaymacana and Tripedalia cystophora. Haeckel’s classification, as given in his “System der Medusen,” is an excellent one and will be followed in this case. One of the new species, however, will not classify under either of Haeckel’s two families, so that for it a new family has been formed and named the Tripedalidæ. In showing the systematic position of the two new forms, an outline of Haeckel’s classification will be given, so far as it concerns our species, together with the additions that have been made necessary. CUBOMEDUSÆ (Haeckel, 1877).