MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 125 



Gonyaulacysta cassidata (Eisenack & Cookson) 

 PI. 14, figs. 3, 4 ; Text-fig. 31 



i960. Gonyaulax helicoidea subsp. cassidata Eisenack & Cookson : 3, pi. 1, figs. 5, 6. 



1962. Gonyaulax cassidata Eisenack & Cookson ; Cookson & Eisenack : 486, pi. 2, figs, i, 2. 



1964. Gonyaulax cassidata Eisenack & Cookson ; Cookson & Hughes : 42, pi. 5, fig. 10 only. 



Emended diagnosis. A Gonyaulacysta with thin-walled, broadly ovoidal theca, 

 surmounted by apical pericoel terminating in short horn. Tabulation 4', ia, 6", 6c, 

 6'", ip, 1"" ; plate boundaries demarcated by high, delicate crests with smooth or 

 denticulate edges. Cingulum strongly spiral, of moderate breadth : sulcus broaden- 

 ing posteriorly, weakly sigmoidal. (A median dorso-ventral plane would barely 

 intersect the two ends of the cingulum.) Surface of shell smooth or only minutely 

 granular : irregular scatter of tubercles present in some specimens. 



Holotype. Specimen P. 17869, National Museum of Victoria, Australia. Lower 

 Cretaceous (Albian) ; Santos's Oodnadatta bore, South Australia, at 327 feet depth. 



Material (figured). Geol. Surv. Colin, slide PF.3047(i). Chalk, H.M. Geological 

 Survey Borehole, Fetcham Mill, Surrey, at 730 feet depth. Upper Cretaceous 

 (Cenomanian) . 



Fig. 31. Gonyaulacysta cassidata (Eisenack & Cookson) . Tabulation. Left, 

 ventral view ; right, dorsal view, x c. 1000. 



