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A developmental and anatomical re-investigation of a clone of this freshwater coccolithophorid previously examined by Braarud (1954) has shown that its tremaliths (the technical term for the type of coccolith produced) are modified scale rims as in the cognate case of the placoliths of Coccolithus pelagicus and the cricoliths of Cricosphaera carterae for which this was already known. The origin of the tremaliths within the cisternae of the Golgi system is traced, together with that of unmineralized scales which accompany them or of which they form part. In addition, the protoplast of Hymenomonas roseola is shown to be distinguished by peculiarities of the haptonema, pyrenoid and flagellar roots, which are matched exactly in Cricosphaera carterae but not yet known elsewhere. Some of the implications of these findings are discussed, and the importance of the protoplast for the taxonomy of coccolithophorids and for the morphological interpretation of coccoliths is stressed.

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