. Bulletin of the British Museum (Natural History), Geology. 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', la, 6", 6c, 6'", ip, i"" : plate boundaries demarcated by moderately high crests, densely and finely perforate, with straight or ragged edges. Cingulum broad, strongly spiral :
. Bulletin of the British Museum (Natural History), Geology. 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', la, 6", 6c, 6'", ip, i"" : 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 (i), Chalk. Survey Borehole, Fetcham Mill, Surrey at 770 feet depth. (Cenomanian). Dimensions. Holotype : overall length 62\l, breadth 49[j. breadth 45- 5 ; length of horn gji. 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 i' 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 i'" being very small and having an ill-defined boundary with the sulcus. The single posterior intercalary plate separates plates i'" and 2'" from the quadrate antapical plate. Geological Upper Cretaceous shell length 53(1,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearanc
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