MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 119 



characteristic trifurcate appearance. There are four apical plates, plate 1' occupying 

 the anterior prolongation of the sulcus and being unusually broad. Six precingular 

 plates are present, the sixth being reduced to accommodate the anterior intercalary 

 plate. 



Six postcingular plates are present, plate 1'" being reduced to accommodate the 

 posterior intercalary plate. A roughly quadrate posterior ventral plate separates 

 the sulcus from the single large antapical plate. 



The cingulum is of moderate breadth and forms a strong laevorotatory spiral such 

 that its two ends differ in antero-posterior position by three times its breadth. It 

 comprises cetainly five, possibly six cingular plates. The sulcus is short and broad. 



The surface is densely granular. In at least one specimen (figured), a very few 

 short spines are present on the surface of the hypotract : spines are not present, 

 however, on the holotype. The degree of granulation of the sulcus is markedly less 

 than that of the rest of the surface. 



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

 seen. 



Remarks. Gonyaulacysta episoma sp. nov. is characterized by its combination of 

 shape, tabulation and crest character. Gonyaulacysta nuciformis (Deflandre 1938), 

 from the Upper Jurassic, has a somewhat similar overall shape and degree of granu- 

 lation, but the shell wall is thicker, the tabulation is less clear and the form of the 

 crests is quite different. Gonyaulacysta scotti (Cookson & Eisenack 1958) an inade- 

 quately described species from the Upper Jurassic of Western Australia, has rather 

 similar crests, but has a more markedly ovoidal shape and an apical horn of dissimilar 

 type. Gonyaulacysta tenuiceras (Eisenack 1958) from the Aptian of Germany, has a 

 horn and crests of somewhat similar character, but the crests are much higher and 

 the tabulation is markedly different. A specimen figured as G. tenuiceras by 

 Alberti (1961, pi. 11, fig. 7), from the Upper Barremian of Germany, may well be in 

 fact G. episoma. 



Gonyaulacysta hadra sp. nov. 

 PL 14, fig. 1 ; Text-fig. 28 



Derivation of name. Greek, hadros, well-developed, bulky, stout — referring to 

 the unusually large size. 



Diagnosis. A Gonyaulacysta with thick-walled, spherical to spheroidal shell and 

 long, tapering apical horn. Tabulation 4', ?oa, 6", 6c, 6'", ip, 1 p.v., 1"" ; poorly 

 marked by crests in form of very low ridges bearing well spaced, extremely abbreviate 

 spinelets. Cingulum weakly spiral : sulcus short, confined to ventral region. 

 Surface generally densely granular. 



Holotype. B.M.(N.H.) slide ¥.51731(1). Speeton Clay, Shell West Heslerton 

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

 (Upper Barremian). 



