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A light and electron microscopical study has been carried out on the morphology and microanatomy of two marine species of Heteromastix and less completely on two freshwater samples from the same genus, one only of which is named; this one is, however, important as the type species of the genus (H. angulata Korsh.). Agreement in salient features indicates that Bipedinomonas N. Carter andAnisomonas Butcher, under which the marine species were previously described, should be discarded as later synonyms of Heteromastix Korsh. Apart from the nomenclatural clarification the most important new findings concern the details of the periplast on cell body and flagella, the presence of stellate scales as well as plate-scales on both types of surface, the presence within the body of a starch shell not giving the normal colour reaction with iodine, and of a characteristic fibrous ‘root’ joining the flagellar bases to the plastid surface. A major finding of electron microscopical interest is the clarity with which the formation of scales and of flagellar hairs has been traced to the Golgi cisternae. These observations are an important addition to previous knowledge concerning genera of related green flagellates possibly referable to the class Prasinophyceae.

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SUMMARY Chlamyclomonas smithii sp. nov. is described from two strains of Chlamyclomonas originally isolated by Dr. Gilbert M. Smith. The mating types are inter‐fertile with those of Chlamyclomonas reinhardtii (Culture No. 89 and 90) maintained in the Culture Collection of Algae at Indiana University. This species is distinguished from all others by the body shape and slit chromatophore.

Strains of Chlamydomonas with cross‐reacting sporangium wall autolysins also correspond in morphology, reproduction, development, and physiological properties. We therefore undertook a revision of the systematics of the genus based on this correlation. Isolates of Chlamydomonas available in culture collections were re‐examined microscopically, preferably with respect to their reproduction and development. In particular all authentic clones, on which earlier species diagnoses had been founded, were included. Their detectable autolysins were tested crosswise in bioassays. We intend to complete and extend species diagnoses in order to separate species more precisely. ‐ At first 7 strains with a common sporangium autolysin have been examined, four of which are the authentic cultures of the species C. aggregate Deason et Bold, C. akinetos Deason et Bold, C. applanata Pringsheim, and C. humicola Lucksch. They have been brought together under the name of the species first described, C. applanata Pringsheim, due to their apparent similarities in morphology and development. An amended species diagnosis is presented. The significance of authentic cultures as important sources of further information is discussed.

SUMMARY Electron microscopy has clarified the structure and taxonomic position of the genus Mesostigma (type species M. viride Lauterborn) by showing that the two unequal flagella are covered with small scales of a characteristic shape and that they arise from a conical pit in the upper surface of the cell which is lined by a dispersed system of tubes or fibres as in the related genus Pyramimonas. The periplast over the rest of the cell is shown to contain two other very distinct kinds of scales which are described. The origin of all three types of scale has been traced to vesicles derived from Golgi cisternae, and scale liberation though not traced in detail is believed to take place into the flagellar depression. The peculiar flattened shape of the chloroplast with restriction of starch storage to a swollen border, the structure of the pyrenoid and the distribution within the cell of other cell components is also indicated in outline. It is concluded that though unique in many respects the genus Mesostigma is unequivocally a member of the Prasinophyceae (sensu Parke & Manton, 1965) and is relatively remote from the Volvocales in which it was formerly classed.