Our knowledge of this remarkable genus of freshwater Oligochæta is at present entirely due to Dr Gustav Eisen. Within a year or two of discovering the type species, Sutroa rostrata, Dr Eisen found a second species, evidently referable to the same genus, which has been named Sutroa alpestris. Examples of both of these species have been most kindly forwarded to me by Dr Eisen; and I therefore take the opportunity of offering a few observations upon the structure of the genus, and upon its relations to other Oligochæta, as I am able, in a few matters, to supplement Dr Eisen's papers.

The account given by Eisen of Sutroa alpestris is evidently based upon a study of the living worm; it is therefore very full as regards the vascular system, but not quite so detailed where it concerns the generative organs, which are more conveniently studied by the section method. It is more especially to these organs that I desire to again draw attention.

Short synopses of my earlier results were given in the Gardeners' Chronicle for April and July 1890. Until after the publication of these I was not aware that some advance had already been made in the direction indicated, and my best thanks are due to Dr Masters for calling my attention to one or two publication's on the subject.

In 1831 Professor J. S. Henslow compared a hybrid Digitalis with its parents in a wonderfully minute way, when we consider the degree to which histology had advanced in his time. He demonstrated that in the size and shape of the hairs and other structures, the hybrid was intermediate between its parents. Wichura and Kerner have proved that the same is true of Willows and Pulmonarias respectively.