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In 19611 I suggested that many post-Paleozoic organic microfossils that had been called hystrichospheres are really dinoflagellate cysts. Further extensive studies fully support that view. Although new findings and some revisions in details of the interpretations offered in 1961 await future publication, I feel it is appropriate now to propose several nomenclatural changes and taxonomic revisions affecting these fossils. That is the purpose of this paper, which includes (following certain background information): (1) an emendation of the dinoflagellate family Hystrichosphaeraceae, (2) proposal of three new dinoflagellate taxa: Hystrichosphaeridiaceae n., fam., Areoligeraceae n. fam., and Achomosphaera n. gen., (3) a recommendation that the term Hystrichosphaerida no longer be used and that its informal variations, such as hystrichosphere, be used cautiously, and (4) a proposal for a new informal group of microfossils of organic composition and unknown affinity to be known as acritarchs. Note on Use of Botanical Code.-Downie et al.2 recently proposed that fossil dinoflagellates and hystrichospheres (including acritarchs in the sense of this paper) be treated nomenclaturally under the Botanical Code. I have adopted this proposal (but without certain of their accessory suggestions regarding the erection of natural genera and form-genera) chiefly on the grounds (1) that, as algae, the dinoflagellates are nomenclaturally botanical entities, and (2) that the acritarchs, whose affinities are unknown (though they, too, probably, include many fossil algae), should be treated under the same code as the dinoflagellates for purely practical reasons. However, this paper is not the place to discuss some of the knotty problems raised by transferring genera and families from the realm of the Zoological Code to that of the Botanical Code; for example, problems that devolve from the distinction between natural genera, organ-genera, and form-genera under the Botanical Code and the effect of this distinction on family names proposed originally under the Zoological Oode. These and other problems are currently under extensive discussion in correspondence among interested co-workers. Slowly, workable solutions to them will emerge. For the present, I have simply used botanical family names in the same manner as zoological family names have been used in the past. Thus, Hystrichosphaeridae becomes Hystrichosphaeraceae, etc. This may not be the ideal procedure, but it seems at least a temporarily acceptable one that is consistent with stability in nomenclature, which is the spirit and purpose of nomenclatural codes. Theca, Cyst and Test.-Currently incomplete research suggests that the fossilized remains of dinoflagellates possibly nowhere include the theca, if that term is used to designate only the external cellulose or cellulose-like covering of a motile dinoflagellate cell. Instead, the fossils seem to represent a layer of considerably more resistant material that formed inside the theca. Probably the structure formed by this resistant layer was functionally a cyst, but it will be referred to herein by the

A DISCUSSION AND PROPOSALS CONCERNING FOSSIL DINOFLAGELLATES, HYSTRICHOSPHERES, AND ACRITARCHS, II