MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 55 



those forms having a cingular region devoid of processes. He concludes that the 

 character of the cingular processes, or their absence, is of taxonomic importance and 

 could be used in the subdivision of this genus. 



During a revision of the genus Hystrichosphaeridium it was noticed that most of 

 the species could be placed in well-defined groups based primarily on the type of 

 archaeopyle, the number of processes on the apical region and the overall number of 

 processes on the central body. Forms possessing an apical archaeopyle and one 

 process per plate may have either one, three or four apical processes, the familiar 

 type of Hystrichosphaeridium possessing four apical processes. Subdivision of this 

 group was made on the presence or absence and type of cingular process. The 

 genus Hystrichosphaeridium is emended to restrict it to forms possessing normal 

 tubular cingular and sulcal processes. These forms possess a process arrangement 

 reflecting a certain tabulation, that is characterized by the type species H. tubiferum 

 (Ehrenberg) — 4'(— 5'), 6", 6c, 5-6'", ip, 1"" and a variable number of sulcal proces- 

 ses. Forms where the cingular processes are absent are placed in a new genus 

 Oligosphaeridium. A third genus, Perisseiasphaeridium gen. nov., is erected to 

 contain forms somewhat intermediate between the previous two genera, possessing 

 cingular and sulcal processes not of the normal tubular type, but smaller and usually 

 closed. 



The genus Litosphaeridium gen. nov., is characterized by the possession of three 

 apical processes and the absence of cingular processes. Finally in this group 

 possessing an apical archaeopyle and one process per plate is the genus Cordon- 

 sphaeridium (Eisenack) which is characterized by having an archaeopyle formed by 

 the loss of a single apical plate. 



Two new genera possessing an apical archaeopyle but more than one process per 

 plate are erected. Polysphaeridium gen. nov., possesses numerous processes all of 

 the same type (Diphyes is easily distinguishable by the presence of a large antapical 

 process) and Tanyosphaeridium gen. nov. possesses an elongate central body and at 

 the antapex probably 3 to 6 antapical processes. 



Two new genera have been erected possessing an epitractal archaeopyle. Homo- 

 tryblium gen. nov. possesses tubular processes of a more or less constant size, there 

 being three apical processes. The presence of three apical processes may indicate a 

 relationship with Litosphaeridium gen. nov., however cingular and sulcal processes 

 are well developed in Homotryblium whereas in Litosphaeridium they are absent or 

 very reduced. The other genus possessing an epitractal archaeopyle is Callaio- 

 sphaeridium gen. nov. which is represented by only one species, C. asymmetricum 

 (Deflandre & Courte ville) . It is extremely distinctive, possessing both solid and 

 tubular processes, one or perhaps two, apical processes and no antapical processes. 



Genus HYSTRICHOSPHAERIDIUM Deflandre 1937 : 68 

 1958. Hystrichosphaeridium Deflandre ; Eisenack : 399, 400. 



Emended diagnosis. Subspherical chorate cysts possessing a reflected tabulation 

 of 4' (-5'), 6", 6c, 5-6'", ip, 1"" and a variable number of sulcal processes. Processes 



