SINCE the publication of a paper1 describing Pellibranchus cinnabareus as a new genus and species of non-pelagic nudibranch mollusc, Dr. Nils H. J. Odhner, of the Riksmuseet, Stockholm, has pointed out to me that Pellibranchus is a synonym of Okadaia Baba2. Okadaia contains small nudibranchs less than 5 mm. in length, limaciform, head and foot not carinate ; rhinophores simple, contractile, without sheaths ; mouth slit-like ; gills absent ; anus and nephroproct opening slightly to the right of the middorsal line, about a third of the way back from the head ; genital orifices obliquely in front of the anus on the right side of the body ; jaw plates absent ; radula formula, 3, 0, 3 ; liver divided into three or four lobes ; true heart not enclosed in a pericardium ; gonads consisting of two or three testes and five or six ovaries ; development is direct. These are the characters of the nudibranch described in my paper, and Pellibranchus is clearly a synonym of Okadaia which now contains Okadaia elegans Baba, 1930, the type from Japan, and Okadaia (= Pellibranchus) cinnabareus (Ralph, 1944) from New Zealand.