As a mere sciolist in geology it sometimes occurs to me to wonder whether enough attention has been given to the distribution of freshwater fauna, in estimating the origin of British land contours and the course of rivers. It is generally accepted that until the North Sea was formed by an extensive subsidence of land in an age not geologically remote, all eastward-flowing rivers south of and including the Yorkshire Ouse were tributaries of the Rhine. The natural result of this should be that the fishes of the Rhine should continue to inhabit its ancient tributaries, as in fact they do in all rivers between and including the Ouse of Norfolk, the Ouse of Yorkshire, and the Trent system. But there is a notable absence from the Thames and the Medway of two species common to the aforesaid rivers and the Rhine—namely, the burbot or eel-pout (Lota vulgaris) and the grayling (Thymallus vulgaris).