no MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



that the fossil Gonyaulax species were in fact cysts. Subsequently Evitt & Davidson 

 (1964) demonstrated from studies of modern dinoflagellates that the genus Gonyaulax 

 formed cysts of more than one type, but certainly including chorate cysts attributable 

 to the genus Hystrichosphaera ; and Sarjeant (1965, text-fig. 3) showed that the 

 arrangement of processes in the chorate Lower Cretaceous species Oligosphaeridium 

 (formerly Hystrichosphaeridutm) vasiforum indicated a Gonyaulax-type motile 

 stage. 



A somewhat confusing nomenclatural situation thus presents itself. On the one 

 hand, it is now clear that a single modern dinoflagellate genus, with a constant 

 tabulation, may form cysts which are of such distinctly different morphology as to 

 merit classification into different form-genera ; it is arguable, on this basis, that the 

 cyst characters indicate different evolutionary lineages and fully justify splitting of 

 the modern genus. On the other hand, should the modern genus be retained 

 unamended, it might well be considered that retention of separate generic names for 

 the fossil cysts, which are simply stages in the life cycle, is unjustifiable. 



The terms of the " International Code of Botanical Nomenclature " recognize the 

 existence of genera of three types — natural " Linnaean " genera ; organ genera, 

 representing either parts of plants or stages in their life cycles ; and form genera, 

 defined on morphology alone. Certain genera of dinoflagellate cysts may prove, as 

 in the case of Hystrichosphaera, to have a determinable relationship to a living genus 

 defined on its motile stage. However, it remains to be proved that a particular cyst 

 type can be produced only by one particular motile type ; it is entirely possible that 

 the same cyst type might be produced by related, but different, motile types. The 

 dinoflagellate cysts, whether recent or fossil, are best treated as form genera and spe- 

 cies, unless or until special provisions are framed for their treatment. 



These problems have been discussed at length by Deflandre (1964), Evitt & David- 

 son (1964) and Norris (1965). A first step towards their solution was taken by 

 Deflandre (1964 : 5) : 



" . . . je place dans le genre Gonyaidacysta nov. gen. (generotype : Gonyaulax 

 jurassica Defl. 1938) toutes les especes fossiles a tabulation de Gonyaulax 

 representees par des theques a cotes saillantes plus ou moins ornementees 

 (pectinees, epineuses, denticulees etc . . .) munie d'un archeopyle (3ieme 

 plaque pre-equatorial)." 



This proposal provides a partial answer to the problem ; however, the fossil species 

 previously classed into Gonyaidax include not only forms with a precingular archaeo- 

 pyle formed by loss of plate 3", but also forms with apical, epitractal and cingular 

 archeopyles. Moreover, the diagnosis remains too wide in terms of tabulation and 

 overall morphology ; forms like Hystrichodinium and Heliodinium would become 

 homonyms, should their tabulation be shown to correspond to that of Gonyaulax. 

 A more restricted diagnosis of Gonyaidacysta is therefore proposed in the section that 

 follows ; the status of the genera mentioned earlier is reviewed ; and new genera are 

 set up to accommodate species which do not accord with the revised concept of 

 Gonyaulacysta, either in tabulation, ornamentation or mode of archaeopyle formation. 



