no MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Remarks. In its possession of an apical pericoel, surmounted by a horn, Gonyau- 

 lacysta axicerastes differs from all other species of the genus except G. cassidata. It 

 differs from G. cassidata in the form of the central shell, which is more ovoid in the 

 latter species ; the less markedly sigmoidal form of the cingulum ; the more slender 

 shape of the apical horn ; the detail of ventral tabulation ; and the possession of an 

 antapical horn. Scriniodinium apaletiim Cookson & Eisenack i960, from the Upper 

 Jurassic of Australia, has an apical horn and pericoel of similar form but differs in 

 having an antapical pericoel also. The tabulation of Scriniodinium apaletum has not 

 yet been fully described. 



Gonyaulacysta helicoidea (Eisenack & Cookson) 



PL 13, figs. 7, 8 ; PL 15, figs. 8, 9 ; Text-fig. 26 



i960. Gonyaulax helicoidea Eisenack & Cookson : 2, pi. 1. figs. 4-9. 



Emended diagnosis. A Gonyaulacysta with spheroidal to ovoidal theca sur- 

 mounted by blunt apical horn. Epitract longer than hypotract ; antapex flattened. 

 Tabulation : 4', ia, 6", 6c, 6'", ip, ?i p. v., 1"" ; plates bordered by denticulate 

 crests, varying considerably both in height and character of denticulation. Cingu- 

 lum strongly spiral : sulcus sigmoidal, plates 6" and 1'" roughly L-shaped. Surface 

 bearing irregular scatter of tubercles. 



Holotype. The specimen figured by Eisenack & Cookson (i960, pi. 1, fig. 4). 

 I. C. Cookson Colin., Melbourne. Lower Cretaceous (Aptian or older) ; Lake 

 Phillipson bore, South Australia, at 87 ft. 10 in. 



Material (figured). B.M.(N.H.) slide ¥.51718(1), Speeton Clay, Shell West 

 Heslerton Borehole No. 1, West Heslerton, Yorks., at 42-5 metres depth, Lower 

 Cretaceous (Lower Barremian). 



Dimensions. Holotype : overall length 78(1, breadth 56jx. Range of Australian 

 specimens : overall length 62— 86fx, breadth 48-67(1. Figured specimen (Speeton 

 Clay) : overall length 45 [jl, breadth 45^ ; shell length 38^, breadth 43^ ; length of 

 horn 7(x. Range of English specimens : overall length 44-53(1, breadth 42-46(1.. 



Description. The shell shape varies from an ovoid to an oblate spheroid ; the 

 length of the horn and the height and form of the crests are also very variable. 



Of the four apical plates, plate 1' occupies the anterior prolongation of the sulcus : 

 it is long and narrow. An elongate anterior intercalary plate is present alongside it. 

 The four apical plates together form the apical horn, their crests being confluent at 

 its tip. Six precingular plates are present, with plate 6" small, elongate, and roughly 

 L-shaped. Six post-equatorial plates are present, plate 1'" being roughly axe- 

 shaped, with broad anterior and narrow posterior portions. A quadrate posterior 



