MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 15 



(6) Chorate Cysts. 



Chorate cysts in general resemble Hystrichosphaeridium and include all the 



typical hystrichospheres. They are distinguished from proximate cysts by the 



greater contraction that took place during cyst formation, for the cyst has only 



. Radius of endocoel 



about I the volume of the motile cell and the contraction ratio „ ,. n 



Radius overall 



is typically about 0-6. As a consequence of this greater contraction the tabulation 

 is often only indirectly determinable, if at all, and the general appearance is not 

 dinoflagellate-like. Furthermore, the outgrowths of the periphragm are longer and 

 often exceed in length the endocoel radius. These outgrowths are commonly spine- 

 like, and are either located from the site of the sutures (sutural appendages) or 

 within the edges of the plates (intratabular appendages). 



Intratabular appendages occur in various ways, but are usually either hollow or 

 grouped in patterns related to the tabulation. The hollow processes are open at the 

 distal end in Hystrichosphaeridium, but in Cleistophaeridutm they are closed distally. 

 So far, no species definitely identified as a dinoflagellate cyst has spines which open 

 into the endocoel. 



The open-ended processes have various kinds of distal openings, which may flare 

 like trumpets or constrict ; various kinds are illustrated in Text-figs. 3, 4. In 

 some cyst groups the processes may be connected distally by narrow solid rods 

 (trabeculae) : in others a thin membrane (ectophragm) may still persist between the 

 distal ends. The ectophragm must have been laid down very close to the motile cell 

 envelope. Often only one appendage occurs on each plate, but there may be more, 

 and when the appendages are very numerous the tabulation may not be determinable 

 at all. 



Other kinds of intratabular appendages are often found to be solid and to occur in 

 groups (see Text-figs. 3, 4). Polystephanephorus is characterized by annulate 

 groups, and Areoligera by coronate and soleate groups. Usually no more than one 

 group is found associated with one plate so that they are useful in determining the 

 tabulation. 



Sutural elements may consist of rows of spines or of spines situated only at plate 

 corners (gonal spines), but commonly these spines have merging bases forming 

 continuous flanges or lists as in the proximate cysts. These elements are usually 

 solid but small pyramidal cavities may develop at the base of the gonal spines, 

 which in Hystrichosphaeropsis begin to run together to form a larger, more 

 continuous cavity. 



Several sub-divisions of the chorate group of cysts are found to be useful. The 

 typical chorate cysts are represented by highly condensed forms like Hystrichosphae- 

 ridium and Cleistosphaeridium with cylindrical or spine-like processes. Their 

 condensation ratio is about o- 5 or o- 6 and the outgrowths are intratabular. Proximo- 

 chorate cysts include forms like Hystrichosphaera which have generally lower conden- 

 sation ratios (o-6-o-8) and sutural outgrowths which more readily indicate the 

 tabulation. Raphidodinium is a highly condensed representative. The trabeculate 



