138 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Description. The apical horn is more or less conical, a bifid appearance being 

 imparted by the apical junction of crests separating the four apical plates. Plate i' 

 is narrow and elongate, occupying the anterior prolongation of the sulcus ; the 

 anterior intercalary plate is also rather elongate. Six precingular and six post- 

 cingular plates are present : plate i'" is very reduced and elongate. A posterior 

 intercalary plate separates plates i'" and 2'" from the single plate occupying the 

 antapex. 



The cingulum forms a laevorotatory spiral such that its two ends differ in antero- 

 posterior position by three times its width : six cingular plates are present. The 

 sulcus is constricted in its median portion, but widens considerably as it approaches 

 the antapex. 



The crests are in part smooth, in part denticulate : they are sometimes very 

 finely perforate. A scatter of tubercles may be present, their number and distribu- 

 tion varying considerably between individuals. 



Plate 3" is consistently lost in archaeopyle formation : its absence is not always 

 obvious in unstained specimens, as a result of the delicacy and tranparency of the 

 shell. 



Remarks. Psaligonyaulax deflandrei sp. nov. is present throughout the Ceno- 

 manian of the Fetcham Mill Borehole, but it is never abundant. Cookson & Hughes 

 (1964) figured a representative of this species from the Cambridge Greensand 

 (PAlbian-Cenomanian) as Gonyaulax cassidata. The latter species, now Gonyau- 

 laesyta cassidata, has a comparable distribution and a similar tabulation : it may be 

 related to P. deflandrei, but intermediate forms are not known. 



OTHER SPECIES 



The species Scriniodinium apaletum Cookson & Eisenack i960, from the Upper 

 Jurassic of Australia and Papua, appears attributable to Psaligonyaulax : its 

 tabulation has not, however, been described to date. 



Following the reconsideration herein of the genera Hystrichosphaeropsis and 

 Rottnestia, the species R. simplicia Cookson & Eisenack 19616, which has poorly 

 developed apical and antapical pericoels and lacks spines arising from crest nodes, 

 becomes Psaligonyaulax simplicia (Tertiary ; Rottnest Island, Australia). 



Genus HYSTRICHOSPHAEROPSIS Deflandre 1935 



Emended diagnosis. Bicavate dinoflagellate cysts, pericoel divided by broad 

 median zone of contact with inner body. Outline typically somewhat angular, 

 apical and antapical periocoels typically quadrate in outline ; apical horn arising 

 from former. Tabulation ?3-4', ?ia, 6", 6c, 6'", ip, 1"", determinable on periphragm ; 

 sutures in form of ridges of varied height and ornamentation ; simple or furcate 

 spines of varied length arising from some or all crest nodes. Surface of endophragm 

 and periphragm smooth, granular, punctate, nodose or reticulate. Precingular 

 archaeopyle formed by loss of plate 3". 



