The new species to be described here is very common in the English Channel, though like other members of this genus it is so fragile that it needs to be cultured to be effectively detected. This particular species has been under observation for some years, but publication has been deferred until sections of it could be made available. The observations involved in the taxonomic description have been based on two early isolates numbered 4 and 43 in the Plymouth collection. More recently, however, it has been encountered frequently in routine sea-water samples brought into temporary culture to record the nanoplankton forms present, and we are therefore able for the first time to give tables of seasonal and depth distributions for the incidence of the species, at various stations. In Table i of Appendix (p. 187) its occurrences are listed, all being from water-bottle samples, except those taken on 23 August 1950 and 6 November 1957 (fine tow-net) and on 19 June 1957 (very fine tow-net). Table 2 (p. 188) gives densities at different depths sampled on one day at Hydrographic Station E 1.