126 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Dimensions. Holotype : overall length &3\i, breadth 52(1. Range of Australian 

 specimens : overall lengths 71 to 9551, breadths 47 to 57^. Figured specimen : 

 overall length 78^, breadth 53^, shell length 42^, breadth 45^. Range of English 

 specimens : overall lengths 60 to 78^ breadths 46 to 53^. 



Description. Shell broadly ovoidal, the apical pericoel and high antapical 

 crests imparting a polygonal appearance. The crests separating the four apical 

 plates converge at the top of the apical horn, which has a bifid appearance. Plate 1' 

 is elongate but relatively short, occupying the anterior prolongation of the sulcus. 

 The apical pericoel comprises the apical plates, the anterior intercalary plate and the 

 anterior portions of the six precingular plates : its volume is between one-third and 

 two-thirds that of the shell proper. The anterior plate is somewhat elongate : 

 plate 6" is reduced and almost triangular. Six postcingular plates are present. 

 Plate 1'" is reduced and linear, difficult to see in many specimens : it is displaced by 

 the posterior broadening of the sulcus. A small, quadrate posterior intercalary 

 plate separates plates 1'" and 2'" from the single, polygonal antapical plate. 



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

 posterior position by over twice its width. It is composed of six plates. The sulcus 

 is of moderate breadth on the epitract but broadens considerably as it approaches 

 the antapex. 



The shell wall and the crests are both delicate. The crests are high, typically but 

 not consistently denticulate, sometimes minutely and irregularly perforate : the 

 very high crests surrounding the antapex appear to be sustained by delicate spines 

 at the angles. Tubercles may be present ; their number and distribution varies 

 considerably between individuals. 



A precingular archaeopyle is formed by loss of plate 3" : in some specimens, 

 including the figured specimen, the cingulum is slightly torn also. 



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

 The English specimens correspond closely to those figured from Australia, differing 

 only in details of shape and proportionate size of shell and pericoel, features in which 

 some degree of variation would be expected. 



Cookson & Hughes (1964) described this species from the Cambridge Greensand 

 (?Albian-basal Cenomanian) : of two forms figured, one (p. 5, fig. 10) corresponds 

 to Gonyaulacysta cassidata as here interpreted, the other (p. 5, fig. 11) appears to 

 belong to Psaligonyaulax deflandrei sp. nov. (p. 137). Both species range right 

 through the Cenomanian horizons of the Chalk of the Fetcham Mill Borehole, albeit 

 consistently in low numbers only. 



Gonyaulacysta whitei sp. nov. 

 PL 14, fig. 2 ; Text-fig. 32 



Derivation of name. Named in honour of two early workers on fossil micro- 

 plankton — Henry Hopley White of Clapham, England, an amateur microscopist 



