122 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Description. Shell broadly ovoidal, with the hypotract rounded. The crests 

 separating the four apical plates converge at the apex. Plate i' occupies the 

 anterior prolongation of the sulcus and is markedly elongate. Six precingular plates 

 are present, with plate 6" very reduced to accommodate an anterior intercalary plate. 

 Seven postcingular plates are present , plates i " ' and 2 " ' being reduced to accommodate 

 a rather indistinctly demarcated posterior intercalary plate. A single polygonal 

 plate occupies the antapex. 



The cingulum forms a laevorotatory spiral such that its two ends differ in antero- 

 posterior position by over three times its width. The number of cingular plates 

 appears to be six, but the crests separating them are poorly marked. The sulcus is 

 of moderate breadth and extends from mid-point on the epitract to the antapex. 



The shell wall is of moderate thickness and consists of two distinct layers. The 

 apical horn is formed by the periphragm, the endophragm showing no outbulge ; 

 the horn is thus hollow and contains what is effectively an apical pericoel. The 

 English specimens have a densely granular surface, but lack tubercles such as are 

 present on the German specimens. 



A precingular archaeopyle, formed by loss of plate 3", is generally present. 



Remarks. The diagnosis is emended to include reference to the tabulation. In 

 his original diagnosis, Eisenack figures two alternative tabulation patterns and 



Fig. 29. Gonyaulacysta orthoceras (Eisenack). Tabulation. Left, ventral view 

 right, dorsal view. x c. 750. 



