MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 121 



Four apical plates are present, jointly forming the apical horn. Plate i' is long 

 and broad, occupying the anterior prolongation of the sulcus. An anterior inter- 

 calary plate could not be determined. Six precingular plates are present, plates i" 

 and 6" being reduced. There are also six postcingular plates, with plate i'" reduced 

 to accommodate the large posterior intercalary plate. A narrow posterior ventral 

 plate separates the polygonal antapical plate from the sulcus. 



The cingulum forms a feebly laevorotatory spiral, its two ends scarcely differing 

 in antero-posterior position. The sulcus is short, extending only over the middle 

 third of the ventral surface : it is widest at mid-point and tapers to anterior and 

 posterior. 



A precingular archaeopyle, formed by loss of plate 3", is present in all specimens 

 seen. 



Remarks. In general form, tabulation and crest character, Gonyaulacysta hadra 

 sp. nov. differs from all described species. 



Gonyaulacysta orthoceras (Eisenack) 

 PL 14, figs. 5, 6 ; Text-fig. 29 



1958. Gonyaulax orthoceras Eisenack : 388, pi. 21, figs. 3-14, pi. 24, fig. 1. ; text-figs. 2, 3. 



J 959- Gonyaulax orthoceras Eisenack ; Gocht : 54, pi. 5, figs. 12, 13. 



1961. Gonyaulax orthoceras Eisenack : Alberti : 6, pi. 11, figs. 1—3. 



1963. Gonyaulax orthoceras Eisenack ; Gorka : 30, pi. 3, figs. 1-4. 



Emended diagnosis. A Gonyaulacysta having an ovoidal theca of moderate wall 

 thickness, with strong, tapering apical horn accounting for about one-fifth to one 

 quarter of overall length. Tabulation 4', ia, 6", ?6c, 7'", ip, 1"" ; plate boundaries 

 outlined by low crests bearing very abbreviate spinelets. Cingulum strongly 

 spiral, of moderate breadth : sulcus broad and extending to antapex. Surface of 

 shell granular to tuberculate. 



Holotype. The specimen illustrated by Eisenack (1958, pi. 21, fig. 5) from 

 Preparation Ob. Apt. no. 32. Aptian glauconitic limestone, Deutschen Erdol A.G., 

 Erdolwerke Holstein, boring Marne, Feld Heide, North Germany, at 761-7 metres 

 depth. 



Material (figured). B.M.(N.H.) slide ¥.51730(3). Speeton Clay, Shell West 

 Heslerton Borehole No. 1, West Heslerton, Yorks., at 19-25 metres depth. Lower 

 Cretaceous (Upper Barremian). 



Dimensions. Holotype : overall length 95fi, breadth 71^ ; length of horn 22[x. 

 Range of German specimens : overall length 70-105 [x. Figured specimen : overall 

 length ioofi., breadth 8ojjl ; length of horn c.20[x ; width of cingulum c.5[x. Range 

 of English specimens ; overall length 80-11 5 y.. 



