134 



MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Remarks. The diagnosis is emended to include mention of the archaeopyle and 

 more detail of other features. Differentiating features from Gonyaulacysta 

 are the smaller number of postcingular plates and the lack of an anterior intercalary 

 plate. A number of species of the latter genus show reduction of the first post- 

 cingular plate : presence or absence of an anterior intercalary plate is then determin- 

 ative. 



Leptodinium alectrolophum sp. no v. 



PL 15, figs. 3-6 ; Text-fig. 34 



Derivation of name. Greek, alectrolophos, cockscomb, in reference to the 

 distinctive crests. 



Diagnosis. A Leptodinium having an ovoidal theca, without apical horn. 

 Tabulation 4', 6", 7c, 5'", ip, 1"" ; plate boundaries bearing high, delicate crests 

 which form slight projections at their points of junction. Sulcus and cingulum both 

 of moderate breadth : sulcus relatively long, extending almost from apex to antapex. 

 Cingular plate 7c roughly diamond-shaped. Apical prominence formed by junction 

 of crests of apical plates. 



Holotype. B.M.(N.H.) slide ¥.51725(1). Speeton Clay, Shell West Heslerton 

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

 (Middle Barremian). 



Paratype. ¥.51725(2). Same locality and horizon. 



Dimensions. Holotype : overall length 56^, breadth 50[x ; shell length 47-511, 

 breadth 42-5(x ; width of cingulum 0.5- 5p.. Paratype : overall length 6i-$\i, breadth 

 54- 5(j. ; shell length 50[x, breadth 46^ ; width of cingulum c.6(jl. Other specimens 

 observed were of intermediate dimensions. 



Fig. 34. Leptodinium alectrolophum sp. nov. Tabulation. Left, oblique dorsal 

 view ; right, oblique ventral view. x c. 800. 



