The Genus Pterosperma (prasinophyceae): Species with a Single Equatorial Ala

The life-history of the genus Pterosperma Pouchet is described. Development of the two phases, motile and non-motile, is detailed and a lunar periodicity in their change over indicated. Only the motile phase can persist independently, the cells reproducing asexually by fission. Sexuality has not been recorded for the genus. Criteria for valid generic and specific separation are discussed. Detailed descriptions, synonomy, distribution records, a key and a comparative table of specific characters are given for seven species, each with a single equatorial ala, including four new species, Pterosperma eurypteron Parke, P. porosum Parke, P. inornatum Parke and P. citriforme Parke.