A HISTORY OF KENT the inner into the outer harbour. In 1882 the surface was covered with them, and among the most striking were various species or varieties of Botryllus. These were not identified at the time, and the few specimens preserved in the Public Museum at Sheffield could not now be identified in a satisfactory manner, but are probably Botryllus schlosseri and polycyclus. Botrylloides rubrum (M. Edw.) or a closely allied species, and very fine specimens of Polyclinum auranthim also occurred. Of simple ascidians Ascidiella aspersa (O. F. Miiller), and I think a few in- dividuals of yf.T;/r§-/>;ra (O.F. M.) were found, and Cionaintestinalis (Linn), of larger size than any seen in Essex or Suffolk, those 6 inches long being common. Straits of Dover. On one occasion I dredged between Dover and Boulogne, and found many fine specimens of Alcyoiuum digitatwn (Linn.) and many sponges, chiefly if not entirely, Halichondria panicea (Pallas). The Thames at Greenhithe and Erith. In 1882 and 1883 I spent a number of weeks on my yacht off Greenhithe and Erith, studying the water of the Thames on behalf of the Board of Works. The greater part of my observations have only an indirect connexion with marine biology, and I describe only a few of my results. Much depends on the very strong tide, which causes a great difference in the character of the water at different times of day, which alternates between fresh and very salt. Few or no sedentary animals are able to live under such conditions. Besides this in some places the material deposited at the bottom at the period of the neap tides is carried away by the much stronger current of the spring tides. The living animals swimming in the water, of which I determined the number per gallon, were those which move up and down in the water, and at high tide many small shrimps were met with, whereas the low water is mainly characterized by the presence of such forms as Cyclops and Daphne. Gammarus occurs in about equal numbers in high and low water. As bearing on the removal of sewage from the water by Copepoda I may say that I found they lived about six times as long when small quantities of human excrement were added to the water as when none was added. Those living in the low water soon died in the high water, and one part of sea water added to two or three of fresh soon proved fatal, which explains what I observed where semi-marine conditions occur along the northern shore of Kent. The Medway. In passing down the Medway from AUington to Sheerness, the conditions are much modified by the great extent to which the tide runs out in the upper part; and the amount of salt in the water where a well-marked maximum in the number of Copepoda occurs appears not to be the same as in the estuaries in Essex and Suffolk. 94
