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SUMMARY This paper deals with taxonomic problems caused by the occurrence in nature of forms of Mallomonas species which are considered to be stages in development. They are called 'imperfect' forms; that is, forms that lack features that have been included in the diagnosis. These individuals might be described as separate species except when intermediate forms are noticed which link them with the perfect form of diagnosed species. The fine details of their scales are the same as in the perfect form. INTRODUCTION Mallomonas is a large genus of the Chrysophyceae, a class of algae with golden green chromatophores. Authors differ in the way they subdivide the Chrysophyceae but the genus Mallomonas is usually placed near such familiar genera as the colonial Synura and less close to Dinobryon. Mallomonas is an elegant flagellate possessing a single flagellum and a cell covered with silica scales whose shape and elaborate markings have proved to be specific in their pattern. The scales may have a rigid prolongation, a spine, and may bear a single mobile hinged bristle which is readily detachable. Certain stages of the organism differ considerably from the mature forms. The develop- ment of such 'imperfect' forms has been traced as far as it could be in the wild and in samples of natural waters kept under observation for a few weeks. These forms may have caused a good deal of taxonomic confusion. Imperfect forms may be found in all Mallomonas species. They are mainly seen during a period of rapid cell division or 'swarming'. I describe a few of the most commonly found forms having selected them chiefly because they might lead to taxonomic confusion. Undoubtedly these forms have their place in the life-cycle of the species concerned, but this must be studied by other methods than those used here. Before the advent of the electron microscope the species were based on the shape and size of the cell together with the character and distribution of the bristles and such information about the scales as the microscope of the day could provide. In 1955 Fott published the first electron micrograph of a Mallomonas scale ; this was followed by a similar publication by Asmund (1955). Thus a whole new range of taxonomic facts was revealed, the scale and its markings becoming the ultimate criterion of the species. In my view the diagnosis of a Mallomonas species without an electron micrograph of the scale is now out of date.

Imperfect Forms and the Taxonomy of Mallomonas