ii 4 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Description. Theca spherical, relatively thin-walled and apparently fragile ; 

 of some 20 specimens seen, all had suffered damage additional to archaeopyle 

 formation. The holotype is the best-preserved, but nonetheless shows a tear in 

 median ventral position. Surface densely granular, the granulation rendering 

 the low crests difficult to distinguish in lateral positions. 



Apical horn slender and relatively long, accounting for about J-th of the thecal 

 length. Its tip is formed by plate 4', its base by plates 1 to 3'. Six precingular 

 plates, with plate 6' reduced to accommodate the anterior intercalary plate. Either 

 six or seven postcingular plates ; presence of the presumed boundary between 

 plates 4" and 5" could not be completely verified in any specimen seen, through a 

 combination of damage and unfortunate orientation. Plates 1'" and ip are both 

 small : plate 2'" is also reduced and does not have a boundary with the antapical 

 plate. The antapical plate has the appearance of being inclined towards the ventral 

 face, but this may simply result from compression. 



The cingulum is of moderate breadth and strongly laevorotatory, its two ends 

 differing in antero-posterior position by about one and a half times its breadth. 

 The number of cingular plates is doubtful. The sulcus is short and of moderate 

 breadth. 



All specimens seen have a precingular archaeopyle, formed by loss of plate 3". 



Remarks. In its combination of overall morphology and tabulation, Gonyaula- 

 cysta palla differs from all other described species. The most closely similar species 

 is G. ambigua (Deflandre), which has a similarly spherical shape and crests of compar- 

 able character, but differs in having no comparable apical horn and a different 

 ventral tabulation. 



Gonyaulacysta axicerastes sp. nov. 

 PL 13, figs. 11, 12 ; Text-fig. 25 



Derivation of name. Greek, axon, axis ; kerastes, horned — hence, axially 

 horned. 



Diagnosis. A Gonyaulacysta having a spheroidal shell with apical pericoel 

 crowned by slender horn arising from periphragm. Second short horn, formed from 

 both membranes, present at antapex. Tabulation ?4', ia, 6", 6c, 6'", ip, 1"" ; 

 plate boundaries bearing high delicate crests. Cingulum and sulcus relatively 

 narrow, sulcus somewhat sinuous and broadening posteriorly. 



Holotype. B.M.(N.H.) slide ¥.51727(1). Speeton Clay, Shell West Heslerton 

 Borehole No. 1, West Heslerton, Yorks., at 39 metres depth. Lower Cretaceous 

 (Middle Barremian). 



Dimensions. Holotype : overall length ji\l, breadth 6o(jl ; shell length 50[z, 

 breadth 52^ ; width of cingulum c.5jx ; length of apical horn, 8(x ; length of antapi- 

 cal horn, 7*5fi.. Other specimens seen too damaged for measurement. 



