Abstract
A new species of hydromedusa from the Antarctic, Rathkea lizzioides sp. nov. is described and illustrated. The species is characterized by its rounded bell-shaped form, being almost as broad as high, the presence of an apical dome and a gastric peduncle, the 4 unbranched oral arms, the number of nematocyst clusters (7 to 18 in specimens 1·5 mm or greater in height), the occasional presence of 1 or more nematocyst clusters near the mouth margin, the 5 to 7 tentacles in each perradial tentacular bulb, the 3 to 5 tentacles in each interradial tentacular bulb, and the ovoid shape of the tentacular bulb. A key to the known species of Rathkea is provided.