Some time since I published in the Transactions of this Society a paper upon Phreoryctes; the present paper is the second of what I hope will be a series of memoirs upon the structure of the Oligochæta Limicolæ; this will be a parallel series to that upon the Oligochæta Terricolæ, which is being published by me in the Quarterly Journal of Microscopical Science. As I took pains in my paper upon Phreoryctes to point out, in accordance with views previously expressed by myself and by others, that it is impossible to draw a hard and fast line between those two groups of Claparède, it may seem rather illogical to retain this classification—if only in the title of a paper; I do so simply as a matter of convenience, and without in the least desiring to revive these old divisions of the Oligochæta; indeed the genus Moniligaster, which is treated of in the present communication, is by most naturalists regarded as an earthworm; in many points it does undoubtedly agree with certain terricolous genera; but as its affinities, into the discussion of which I shall enter later, seem to me to be more with the Lumbriculidæ, I put it into the Limicoline series; it is useful to have a name corresponding to “earthworm” for those Oligochæta which are not earthworms, and are for the most part aquatic, and I therefore use that term.