MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 105 



CONCLUSIONS 



The characteristics and known stratigraphical distribution of the 10 genera here 

 considered are summarized in the accompanying Table. At present, the strati- 

 graphic distribution of the genera cannot be correlated coherently with the variation 

 in structure. As more species are attributed to these genera and the stratigraphic 

 range of each genus becomes better known perhaps evolutionary relationships will 

 become more obvious. 



At present no species belonging to these genera have been found earlier than the 

 Middle Jurassic. Three genera are present in the Upper Jurassic ; Polysphaeridium, 

 perhaps a primitive form, bearing numerous processes, and Hystrichosphaeridium 

 and Oligosphaeridium both reflecting similar tabulations and possessing 4 apical 

 processes. From the latter two genera perhaps are derived the remaining genera 

 with the exception of Diphyes. After the Jurassic, although the tabulation remained 

 basically the same, the number of apical plates appears to have become more 

 variable and is probably an important systematic character. The genera Diphyes, 

 Cordosphaeridium and Homotryblium have not been recorded, with certainty, from 

 the Mesozoic. 



Dinoflagellate cysts belonging to the genera discussed are relatively abundant 

 from the Upper Jurassic to the end of the Eocene, but appear to become less common 

 thereafter. After more detailed morphological studies have been performed their 

 importance to world-wide stratigraphy should be considerable. 



