MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



127 



who described assemblages from the Chalk in 1842-44, and M. C. White of the 

 United States, who described in 1862 the first Palaeozoic assemblages, from New 

 York State. 



Diagnosis. A Gonyaulacysta having an ovoidal theca with short, blunt apical 

 horn formed from both shell layers. Tabulation 3', ia, 6", 6c, 6'", ip, 1"" : plate 

 boundaries demarcated by moderately high crests, densely and finely perforate, with 

 straight or ragged edges. Cingulum broad, strongly spiral : sulcus sigmoidal, of 

 moderate breadth but widening somewhat posteriorly. (A median dorso-ventral 

 plane would barely intersect the two ends of the sulcus.) Surface of shell smooth or 

 only very minutely granular. Crests bounding antapex supported at the angles by 

 strong, broad spines. 



Holotype. Geol. Surv. Colin, slide PF. 3048(1), Chalk. 

 Survey Borehole, Fetcham Mill, Surrey at 770 feet depth. 

 (Cenomanian). 



Dimensions. Holotype : overall length 62[x, breadth 49^ : 

 breadth 45-5 ; length of horn 9jx. 



Description. Shell rotund, lemon-shaped, the apical horn joining the shell so 

 smoothly as to have no precise base. Only three apical plates, the crests bounding 

 them forming a pimple on the tip of the apical horn. Plate 1' is elongate and occu- 

 pies the anterior prolongation of the sulcus : its posterior boundary was not clearly 

 determined. An anterior intercalary plate and six precingular plates are present, 

 plate 6" being reduced and subtriangular. Six postcingular plates are present, 

 plate 1'" being very small and having an ill-defined boundary with the sulcus. The 

 single posterior intercalary plate separates plates 1'" and 2'" from the quadrate 

 antapical plate. 



H.M. Geological 

 Upper Cretaceous 



shell length 53[jl, 



Fig. 32. Gonyaulacysta whitei sp. nov. Tabulation. Left, ventral view 

 right, dorsal view. x c. 1000. 



