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A study of clonal cultures of the foraminifer Rosalina floridana (Cushman) was undertaken in order to ascertain the potential for variation of the species and to clarify its taxonomic situation.

The variation potential of this foraminifer is quite high, for some variants could be classified within three different genera (Discorbis, Rosalina, and Planorbulina) of three different families (Discorbidae, Rosalinidae, and Planorbulinidae) and possibly a fourth (Cibicididae). Both, quasi- planispiral and flattened porcellaneous forms were observed to occur. In addition, factors such as sutural angulation, chamber lobation and apertural position, among others, were noted. Light transmitting properties of tests varied, ranging from hyaline through milky white to opaque porcellaneous forms. Reproduction was unimodal, i.e. by repeated schizogony.

Comparison of R. floridana and R. leei indicates that the criteria outlined to differentiate the two are inadequate and that the two probably belong to a single species, R. floridana; R. leei should be considered valid only at the sub-specific level.

The diagnostic criterion of septal wall structure normally used to differentiate the genera Discorbis and Rosalina is in need of extensive examination and re-evaluation of its validity, with consequential taxonomic revisions.

The scanning electron microscope is an extremely good instrument for a study such as this, but it suffers from some defects and in order to achieve complete evaluation of samples, it continues to be necessary to make observations with the compound and dissecting microscopes with transmitted and reflected light.

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Supervisor: Franco Medioli