The medusa fauna of the mid North Atlantic Ocean between 24° and 55° N along approximately 30° W is studied. Two Hydromedusae new to science, Oceania tydemani and Annatiara lempersi, are described. The differences in fauna south and north of 42° N, and faunal borders near 50°, 48°, 45°, 35°, 30°, and 34°-37° N are discussed. The vertical distribution and for some species the diurnal migration and subtropical submergence are recorded.