Milne Edwards (1841) established the genusLeptoclinum for those Didemnidae which possessed a largely expanded common cloacal system. His type of the genus wasL. maculosum which is now placed in the genusDidemnum Savigny (1816). Other species wereL. asperum andL. durum which are now regarded as mere forms ofDidemnum maculosum (e.g. Harant & Vernièeres, 1933); Leptoclinum fulgens which is closely related (see Harant & Vernièeres), if not a mere colour form ofDidemnum maculosum], Leptoclinum listerianum which has been transferred to the genusDiplosoma Macdonald; andLeptoclinum gelatinosum. This last has often been considered synonymous withDiplosoma listerianum, but my reasons for disagreeing with this assumption are elsewhere expressed (Carlisle, 1953). Milne Edwards did not of course describe the course of the sperm duct by whichDiplosoma is distinguished fromDidemnum. In the absence of a description of this diagnostic character we are reduced to a consideration of numerous details of the anatomy of Leptoclinum gelatinosum in order to decide where to place it in the classificatory scheme. When such a point-by-point comparison of shape and curvature of the gut, position of anus, form of atrial and buccal apertures, nature of test, etc., is made, it appears that the one point in which the drawings and descriptions given by Milne Edwards differ betweenL. gelatinosum andDidemnumgelatinosum Mime Edwards (1841) is the presence of a common cloacal system in the former and its absence in the latter. But the presence or absence of a common cloacal system is no longer regarded as a generic distinction (see, for example, van Name, 1945).