MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



149 



smooth, denticulate, or bearing spines of varying lengths ; alternatively sutures 

 marked by lines of closely set spines. Shell surface smooth, granular, punctate or 

 tuberculate. Archaeopyle formation by loss of single apical plate. 



Type species. Microdinium ornatum Cookson & Eisenack 1960a. ?U. Aptian- 

 Turonian : Australia. 



Remarks. At the time when it was proposed, this genus was technically invalid, 

 since its tabulation falls within the limits specified for Gonyaulax. However, 

 Microdinium is now recognized to be a cyst genus and, regarded as such, becomes a 

 valid entity, since its tabulation does not accord with that of Gonyaulacysta either in 

 the original definition of Deflandre or as here emended. 



Representatives of this genus are relatively frequent in the English Cenomanian. 

 The generic diagnosis is emended to accommodate variations in morphology exhibited 

 by these forms. 



Microdinium differs from the majority of fossil dinoflagellate cysts in having an 

 apical archaeopyle ; from Meiourogonyaulax and Xiphophoridium in having only a 

 single apical plate ; and from Glyphanodinium in having six precingular plates. 



Microdinium cf. ornatum Cookson & Eisenack 

 PL 16, figs. 3-6 ; Text-fig. 38 



1960a. Microdinium ornatum Cookson & Eisenack : 6, pi. 2, figs. 3-8, text-figs. 2-4. 



Material (Figured). Geol. Surv. Colin, slide PF.305o(i). Chalk, H.M. Geologi- 

 cal Survey Borehole, Fetcham Mill, Surrey, at 670 feet depth. Upper Cretaceous 

 (Upper Cenomanian). 



Dimensions. Figured specimen : length 40^, breadth 32^. Range length 

 c.30-45fx, breadth c. 22-35^. 



Fig. 38. Microdinium cf. ornatum Cookson & Eisenack. Tabulation. Left, 

 ventral view ; right, dorsal view (the apex is missing), x c. 1250. 



