MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 135 



Description. Theca ovoidal, an appearance of polygonality being imparted by 

 the crests. The pericoel surface bears a coarse granulation, granules being rather 

 regularly spaced. The crests are thin and high, with minute, inconspicuous perfo- 

 rations. Their distal edge is usually smooth, occasionally finely denticulate ; 

 crests of a single individual may in part have smooth, in part denticulate, edges. 



The apical prominence appears to be formed solely by the junction of crests 

 bounding the four apical plates ; there appears to be no apical horn as such. Apical 

 plate 1' occupies the anterior extension of the sulcus ; since the sulcus is long and 

 approaches the apex, plate 1' is proportionately small. The six precingular plates 

 are all quite large. Presence of an anterior intercalary plate was suspected but not 

 confirmed ; if present, this plate is relatively small. Five well-developed post- 

 cingular plates are present ; plate 1'" is reduced to accommodate a quadrate 

 posterior intercalary plate. The antapex is occupied by a single, large plate. 



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

 antero-posterior position by roughly twice its width. There are six clear cingular 

 plates, plus a seventh, diamond-shaped plate, here designated 7c, which lies between 

 the posterior end of the cingulum and the sulcus. The sulcus is narrow and long, 

 extending almost from apex to antapex. 



An archaeopyle is not present in the holotype ; however, the paratype has a 

 well-developed precingular archaeopyle formed by loss of plate 3". 



Remarks. Leptodinium alectrolophum differs from all described species in its 

 combination of crest and thecal morphology and tabulation. It resembles Gonyau- 

 lacysta gongylos in having a seventh polygonal plate at the posterior end of the cingu- 

 lum, but differs in tabulation and crest form and shape. Heslertonia heslertonense 

 has even higher crests ; the surface lacks similar granulation and the tabulation is 

 again different. 



In certain orientations, the high crests of this species produce a misleading 

 resemblance to Scriniodinium : however, detailed study shows that no pericoel is 

 present. 



The form from the Aptian of Germany, figured by Eisenack (1958, pi. 22, fig. 4) as 

 7Gonyaulax sp., may well be attributable to this species. 



Presence of only five postcingular plates presumably results from the trend of 

 reduction of plate 1'" ; thus plate 1'" of Leptodinium corresponds to plate 2'" of 

 Gonyaulacysta. 



other species 



The following species are retained in this genus and accord with the emended 

 diagnosis : 



Leptodinium arcuatum Klement i960. Upper Jurassic (Upper Oxfordian); 

 Germany. 



Leptodinium maculatum Cookson & Eisenack 19616. Eocene ; Rottnest Island, 

 Western Australia. 



