1. Two new schizogregarines are described from the gut of the polychaete worm Potamilla reniformis (O.F.M.). One of these is meanwhile retained in the genus Selenidium Giard as S. potamillae n.sp. For the other a new genus, Meroselenidium, is proposed, of which M. keilini is the type species. The life cycle has been followed in both gregarines.
2. The schizogony of Meroselenidium is characterised by the formation of cytomeres preliminary to the liberation of merozoites. In Selenidium potamillae the schizogony is more like that in S. mesnili and S. caulleryi, though there is a considerable residual body.
3. The gametocytes become attached by their posterior ends. In S. potamillae there is often a multiple association. The gamete nuclei are formed within the nuclear area of the gametocytes. Further development takes place in sea water. Isogametes have been observed in S. potamillae.
4.The gametocyst of Meroselenidium keilini is elliptical in outline and measures about 70 × 55μ. It contains some twenty relatively large spores, 27 × 15μ, the walls of which are transversely ridged; the spores are polyzoic. The gametocyst of S. potamillae is of much the same shape, but only about half the size. The sporocysts are very numerous, spherical, smooth-walled, and measure about 8·5μ in diameter. In neither species does the gametocyst contain any residual mass.
We take this opportunity of thanking the High Commissioner of India and the Delegacy of King's College for grants-in-aid received by one of us (H. N. R.) during the course of these investigations.