A critical examination of the biology and morphology of two species of Cucurbitella, six species of Difflugia and three species of Netzelia is presented. A new term ‘necklace’ is given to species of Difflugia which have an apertural rim, but lack a distinct collar like those of Cucurbitella. Details of the shell surface composition is used to differentiate some species, and a knowledge of the general biology assists the interpretation of these results. Biometrical data is also shown to be useful in separating morphologically similar species. Cytoplasmic features such as the size and structure of the nucleus and nucleolus is used to further distinguish species. As a consequence of our investigation one new species is erected and several synonyms proposed.