MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 147 



spiral, laevo-rotatory. Apical and antapical horns lacking. Sutures marked by 

 high crests bearing long, dagger-like spines, broadly spaced ; crests bordering 

 cingulum especially high and prominent. Surface smooth, granular, punctate or 

 tuberculate. Archaeopyle formed by loss of apical plates ; not all individuals have 

 an archaeopyle. 



Type species. Hystrichodinium alatum Cookson & Eisenack 19626. Cretaceous 

 (PUpper Aptian-Cenomanian) ; Western Australia. 



Remarks. This genus comprises forms with high, pronouncedly spinose crests. 

 It is distinguishable from Heliodinium, Hystrichodinium and Ctenidodinium by the 

 mode of archaeopyle formation, from Heliodinium and Hystrichodinium also by the 

 fact that the sutural spines arise from crests ; and from Ctenidodinium also by the 

 fact that the cingulum is bordered on both sides by high crests. 



Xiphoridium alatum (Cookson & Eisenack) 

 PL 16, fig. 11 



19626. Hystrichodinium alatum Cookson & Eisenack : 478, pi. 2, figs. 1-4. 



Emended diagnosis. A Xiphophoridium having an ovoidal to nearly globular 

 shell, shell wall thin. Tabulation ?4', ia, 6", 6'", ?op, 1"" ; plates bounded by very 

 high crests bearing long, dagger-like spines, crest curving inwards between bases of 

 spines. Cingulum weakly spiral, of moderate breadth and bordered by especially 

 high crests ; sulcus also of moderate breadth, extending to antapex. Shell surface 

 not, or only minutely, granular, bearing numerous tubercles, sometimes apparently 

 arranged to plate margins, generally without obvious arrangement. Apical archaeo- 

 pyle present. 



Holotype. Specimen no. P. 21272, National Museum, Victoria, Australia. 



Material (figured). Geol. Surv. Colin, slide PF.305i(i). Chalk, H.M. Geological 

 Survey Borehole, Fetcham Mill, Surrey, at 840 feet depth. Upper Cretaceous (basal 

 Cenomanian). 



Dimensions. Holotype : overall length 125^, breadth 96^ : length of shell yo\i, 

 breadth 52^. Range of Australian specimens : overall lengths ioo-i02[x, overall 

 breadths 92-iooji.. As a result of orientation or preservation, it did not prove 

 possible to obtain detailed measurements of the English specimens, but dimensions 

 appear comparable. 



Description. This species occurs infrequently throughout the English Ceno- 

 manian, some 20 specimens having been encountered. The specimen figured, seen 

 in terminal view, was the best-preserved and allowed determination of the tabulation : 

 all other specimens were damaged or distorted to some extent. It was not possible 

 to prepare a satisfactory figure. 



