168 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Material (figured). Geol. Surv. Colin, slide PF.3044(i). Lower Chalk, H.M. 

 Geological Survey Borehole, Fetcham Mill, Surrey, at 810 feet depth. Upper 

 Cretaceous (Cenomanian). 



Dimensions. Figured specimen : diameter of central body 32 by 4ijx, length of 

 processes up to 9jx. Range : diameter of central body 20-45 ja, length of processes 

 up to 9(x. Number of specimens measured, 30. 



Remarks. As with Cookson & Hughes (1964), difficulty was met with when 

 trying to distinguish C. ancoriferum from Chlamydophorella nyei (Cookson & Eisenack 

 1958), since the outer membrane and apical prominence of the latter are often obscure. 

 The processes of C. nyei, however, are finer and shorter than those of C. ancoriferum. 



Cookson & Eisenack (1960a) comment on the " transparent tips " of the processes 

 of C. ancoriferum. The cavities are in fact closed by a thin, tranparent membrane. 

 This character may well indicate a close relation to Chlamydophorella. The species 

 Cleistosphaeridium ancoriferum may have arisen by the progressive restriction of a 

 formerly continuous membrane ; or alternatively, Chlamydopherella may have 

 arisen by the extension of a membrane which originally merely tipped the processes. 



Cleistosphaeridium heteracanthum (Deflandre & Cookson) 



PL 2, figs. 6, 7 



1955. Hyslrichosphaeridium heteracanthum Deflandre & Cookson : 276, pi. 2, figs. 5, 6 ; text- 

 figs. 40, 41. 



1961a. Hystrichosphaeridium heteracanthum Deflandre & Cookson ; Cookson & Eisenack : 73, 

 pi. 12, fig. 14. 



1963. Baltisphaeridium heteracanthum (Deflandre & Cookson) Downie & Sarjeant : 91. 



Remarks. The Cenomanian specimens from England are very similar to the 

 forms illustrated by Deflandre & Cookson (1955) from the Upper Cretaceous of 

 Victoria, Australia. 



The surface of the central body may be smooth or reticulate. The processes are 

 extremely variable in shape but do not vary markedly in length. One complete 

 specimen has been found and this possessed one large distinctive process. In all the 

 other studied examples this process was absent and there was, in every case, a large, 

 well defined, archaeopyle. It is probable, therefore, that the process is apical and that 

 the archaeopyle, when developed, is also apical in position. Alignment of the proces- 

 ses on the surface of the central body has not been observed. Some difficulty was 

 experienced in distinguishing C. heteracanthum from C. multifurcatum (Deflandre). 

 The processes of the latter, however, appear to be considerably less varied, most of 

 them terminating with a simple bifurcation or being blunted. 



C. heteracanthum is found throughout the Cenomanian of England, and in Australia 

 it has been recorded from the Upper Cretaceous and Lower Eocene. 



One specimen of C. heteracanthum illustrated by Deflandre & Cookson (1955, 

 pi. 12, fig. 14) appears to possess an apical archaeopyle. For the latter reason this 

 species is tentatively placed in the genus Cleistosphaeridium. 



