150 



MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Description. This form occurred in moderate abundance in the upper levels of 

 the Cenomanian of the Fetcham Mill Borehole, over 20 specimens having been 

 seen ; the apex was missing in all specimens seen. 



The shell is ovoidal in shape, with a somewhat flattened antapex and an apex 

 truncated by the archaeopyle. Six precingular and six postcingular plates are 

 present. The presence of an anterior intercalary plate was suggested in some 

 specimens, but could not be confirmed as a result of distortion around the archaeopyle. 

 Plates 1'" and 2'" are reduced to accommodate a large posterior intercalary plate ; 

 this is of comparable size to the single antapical plate, some specimens indeed give 

 the impression of having two antapical plates. 



The cingulum is very broad and not hollowed ; it is weakly spiral, laevorotatory, 

 its two ends scarcely differing in antero- posterior position. The sulcus is broad 

 throughout its length, but broadest as it approaches the antapex : ventral plates are 

 absent. 



The shell surface is generally quite smooth, but bears a scatter of tubercles, some 

 of which are aligned parallel to sutures ; the number and arrangement of tubercles 

 varies between individuals. The sutures bear closely set spines, capitate and of 

 constant length, giving almost the impression of perforate crests : in vertical view, 

 these give the impression of a string of beads. 



Remarks. These English Cenomanian forms generally closely resemble Micro- 

 dinium ornatum as described from Australia, but differ in two details — the absence of 

 a plate separating the posterior end of the cingulum from the sulcus, and the form of 

 the crests, which are constantly in the form of closely set spines. It is highly 

 probable that they fall within the range of variation of M. ornatum, since they accord 

 closely with the photographs and since Cookson & Eisenack state (p. 7) that " the 



?ia 



Fig. 39. Microdinium setosum sp. nov. Tabulation. Left, ventral view ; 

 right, dorsal view, x c. 1250. 



