In the autumn of 1877 I found on stones in a sea-water aquarium in Professor Huxley's laboratory at South Kensington, the curious little Alga which forms the subject of the present paper. I have ever since kept specimens growing, carried them with me to various places in Britain and on the Continent, and subjected them to frequent examination at all seasons of the year. Dr Bornet has kindly identified it for me as a species of Enteromorpha.