12 



MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



The term " ambitus " applied to dinoflagellates refers to the test outline viewed 

 from the dorsal or ventral side. 



(2) The Morphology of Motile Stage Thecae. 



Modern dinoflagellates may be thin-walled or may have robust cellulosic tests 

 (thecae), which are clearly divided into fields by sutures in the tabulate genera. 

 Text-fig. 2A shows a test of this type and indicates the terminology used to describe 

 it. Other morphological structures are shown in Text-figs. 3, 4. 



(3) Fossilized Motile Thecae. 



Very few fossil dinoflagellates could be considered as the remains of motile stage 

 thecae. The strongest claims can be made for some species of Peridinium and 

 Gymnodinhim from the Upper Cretaceous which lack archaeopyles and show no other 

 structures characteristic of cysts. 



Fig. 2. The tabulation of a modern dinoflagellate, compared with that of a proximate 

 dinoflagellate cyst, a, Gonyaulax polyedra, a dinoflagellate of present day warm and 

 temperate seas. After Kofoid. b, Gonyanlacysta jurassica, from the Upper Jurassic ; 

 a proximate cyst with a precingular archaeopyle. [After Sarjeant (1965) reproduced by 

 permission of the Editor of " Endeavour ".] 



