The species nigra was originally reported by me (BABA, 1937) on a single specimen towed in the Zostera zone in Tomioka Bay, Amakusa, with a presumption that it belonged to Alderia Allman, 1846, in the general organization (depressed body-form and radula) of the animal. At the same time it was noticed that nigra differed from modesta (Lovén, 1844), type of the genus, in a series of such characters as the formation of the rhinophores, the arrangement of the branchial papillae, and especially in the non-terminal position of the anus. The real situation of nigra in the systematics has been obliged to remain unclarified until now. Here I intend to refer to Dr. A. Inaba who kindly afforded me with chances to re-discover specimens of this rare species in fair numbers from among the bushes of Zostera in the vicinity of the Mukaishima Marine Biological Station. Immediately after the above finding the species was confirmed by me from above the leaves of this eel-grass in Osaka Bay. By examining the newly obtained material before me, I have been led to point out additional differences between nigra and modesta in the habitats, in the egg-masses, and in several points of the internal anatomy. Establishment of a new taxon of generic rank thus appeared to be necessary for the sake of the former species (cf. MARCUS & MARCUS, 1956, p. 16; HAND & STEINBERG, 1955, p. 26).