54 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



pyles " as he called them, were not irregular ruptures of a cyst wall but within a 

 species were of constant shape and size. In forms that are definitely cysts of 

 dinoflagellates, archaeopyles are formed by the loss of areas which correspond to a 

 single plate or plates of the original dinoflagellate theca. Four types of archaeopyle 

 were differentiated by Evitt. They are the precingular and intercalary archaeopyles, 

 each formed by the loss of one plate ; the apical archaeopyle formed by the loss of 

 the apical region, commonly four plates ; and the epitractal archaeopyle formed by 

 the loss of the whole of the epitract above the girdle. Examination of species from 

 the genus Hystrichosphaeridium reveals that the archaeopyle is practically always 

 apical for three reasons : 



(i) Surrounding the archaeopyle, in forms possessing less than 30 processes, 

 there are constantly six processes corresponding to six precingular plates ; 



(2) The detached operculum frequently bears four processes, reflecting the 

 four apical plates found in many dinoflagellates ; 



(3) In ovoidal or elongate forms the archaeopyle is usually formed at one of the 

 extremities. 



It was discovered in a number of forms, for instance H. tubiferum (Ehrenberg), that 

 there was an obvious circular arrangement of the processes around the central body 

 of the cyst, and that the number of processes was equal to the number of plates 

 possessed by forms having a typical Gonyaulax-type tabulation. Thus in the above 

 forms, one process on the central body of the cyst reflects one plate in the dino- 

 flagellate theca, the process extending from the cyst to the centre of the plate. 

 Such processes are referred to as being intratabular and from them may be calculated 

 the original tabulation of the dinoflagellate. 



Apart from the apical archaeopyle other features may be used in the orientation of 

 the specimen and the elucidation of the process arrangement. Although the 

 processes may all be of the same type, the sulcal processes, like the corresponding 

 plates, are usually relatively small in size. The ant apical process and the cingular 

 processes usually expand to some extent and terminate with a spinous or serrate 

 margin. The spines become finer away from the process margin and it is very 

 easy to imagine a plate affixed to such a process termination. 



All the above points seem to indicate conclusively that forms belonging to the 

 genus Hystrichosphaeridium are cysts of dinoflagellates. The cysts are formed 

 within dinoflagellate thecae, during or as a response to adverse conditions, the 

 processes acting as pillars between the cyst and the thecal wall, and holding the cyst 

 in position. Subsequently to the cyst formation the dinoflagellate theca is lost, the 

 latter only very rarely being observed in preserved material. Sarjeant (1965, 

 text-fig. 3) tentatively reconstructed the original tabulation of a dinoflagellate theca 

 from the distribution of the processes for the species, Oligosphaeridium vasiformum 

 (Plate 9, fig. 7). 



Evitt (1961) suggests that there were three main divisions in the genus Hystri- 

 chosphaeridium : those forms possessing cingular or girdle processes of similar form 

 to the other processes, those with distinct, often more slender, cingular processes and 



