In supervising the preparation of the plates that illustrate this genus I have impressed upon the artist the importance of representing what he saw rather than what he thought he ought to see, and the figures are very close copies of actual specimens. I have deliberately chosen for representation specimens of _Plumatella_ preserved by the simple methods which are often the only ones that it is possible for a traveller to adopt, for the great majority of naturalists will probably have no opportunity of examining living specimens or specimens preserved by special methods, and the main object, I take it, of this series is to enable naturalists first to distinguish the species described and then to learn something of their habitat and habits. GEOGRAPHICAL DISTRIBUTION.--Of the seven species included in this key five have been found in Europe (namely _P. fruticosa_, _P. emarginata_, _P. diffusa_, _P. allmani_, and _P. punctata_), while of these five all but _P. allmani_ are known to occur in N. America also. _P. javanica_ is apparently peculiar to the Oriental Region, while _P. tanganyikæ_ has only been taken in Central Africa and in the Bombay Presidency. TYPES.--Very few of the type-specimens of the older species of _Plumatella_ are in existence. Allman's are neither in Edinburgh nor in London, and Mr. E. Leonard Gill, who has been kind enough to go through the Hancock Collection at Newcastle-on-Tyne, tells me that he cannot trace Hancock's. Those of the forms described by Kraepelin are in Hamburg and that of _P. tanganyikæ_ in the British Museum, and there are schizotypes or paratypes of this species and of _P. javanica_ in Calcutta. The types of Leidy's species were at one time in the collection of the Philadelphia Academy of Science. BIOLOGY.--The zoaria of the species of _Plumatella_ are found firmly attached to stones, bricks, logs of wood, sticks, floating seeds, the stems and roots of water-plants, and occasionally to the shells of molluscs such as _Vivipara_ and _Unio_. Some species shun the light, but all are apparently confined to shallow water.