148 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



The shape of the archaeopyle is that of a ragged polygon with a narrow sulcal 

 notch, suggesting that four apical plates were originally present. Six precingular 

 plates are present, plate 6" being reduced to accommodate a quadrate anterior inter- 

 calary plate. Six postcingular plates are present ; no posterior intercalary plate 

 could be distinguished in the specimens studied. The antapex is occupied by a 

 single polygonal plate. 



The cingulum is weakly spiral, laevorotatory ; its structure was not clear in the 

 English specimens, but the photographs of the Australian specimens suggest a 

 division into cingular plates (?6). 



The crests are delicate, not or only minutely perforate : the spines arise simply as 

 outgrowths of the crests and are somewhat flattened and dagger-like. The shell wall 

 appears to lack ornament other than the tubercles. 



Remarks. In their original description of this species, Cookson & Eisenack 

 noted the clear presence of tabulation and recognized this as a separating character 

 from other species of Hystrichodinium, in which they then placed it. They do not 

 record the presence of an apical archaeopyle but this feature is suggested in one of 

 their figures (pi. 2, fig. 2). The diagnosis is here emended to include reference to the 

 tabulation and the possession of an apical archaeopyle. 



Genus BELODINIUM Cookson & Eisenack 19606 : 249 



Type species. Belodinium dysculum Cookson & Eisenack 19606. Upper 

 Jurassic (Tithonian) ; Australia. 



Remarks. This genus was diagnosed in the following terms : 



" Shell elongate, unequally divided by a circular girdle. Main body marked 

 into fields by delicate ledges : epitheca with a hollow membraneous horn, 

 hypotheca with a flattened membraneous expansion." 



In the absence of a full knowledge of the tabulation, distinction of this genus depends 

 on the circular nature of the cingulum and the presence of an antapical pericoel. The 

 description of the type species (Cookson & Eisenack 19606 : 250) makes it clear that 

 archaeopyle formation is by loss of the apical plates. The authors state that their 

 interpretation of the genus is " provisional " and " incomplete " : a fuller study of 

 the genus and a revision of the diagnosis are clearly necessary before its status can be 

 validly assessed. 



Genus MICRODINIUM Cookson & Eisenack 1960a : 6 



Emended diagnosis. Proximate dinoflagellate cysts, spheroidal to ovoidal in 

 shape and usually small. Epitract smaller than hypotract. Tabulation 1', o-?ia, 

 6", 6c, 6'", ip, 1"", with differentiation in some species of additional plates in ventra, 

 region. Cingulum broad, weakly spiral ; sulcus broad, extending from apex to 

 antapex. Sutures bearing crests in form of low ridges (perforate or imperforate), 



