MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



57 



Description. Central body composed of thin smooth endophragm and smooth 

 or slightly granular periphragm, the latter also forming the processes. The processes 

 are tubiform with distally a denticulate to serrate circular margin. At the base of 

 each process is a characteristic circular mark caused by the initial divergence of the 

 endophragm and periphragm. The processes, up to 30 in number, are usually 

 shorter in length than the small diameter of the central body. They are of unequal 

 width, the sulcal processes being finer and usually shorter. Lejeune-Carpentier 

 (1940), after examination of the type material, records that this species is common 

 from the Upper Turonian and Senonian. 



The processes of the London Clay specimens often possess unusual foliaceous 

 outgrowths emanating from the margin and these may be diagnostic enough to 

 differentiate Eocene forms from Upper Cretaceous forms. All the London Clay 

 forms have at least 20 processes. 10 specimens were measured, the diameter of the 

 central body being 28-53fx and length of processes i^-2g\i. 



H. tubiferum is uncommon in the Cenomanian of Fetcham Mill (Surrey) and is 

 rather variable in the form of its processes. Specimens which closely resemble the 

 holotype have been observed, the sulcal processes, however, being less noticeably 

 small. At the base of the Cenomanian the processes of H. tubiferum expand distally 

 and terminate in a denticulate margin, a few short secae are often present. Towards 

 the top of the Cenomanian the processes do not expand so much and the secae are 

 more pronounced and isolated. Diameter of central body 30-51(1, length of processes 

 15— 37(x, 26 specimens being measured. 



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Fig. 13. Hystrichosphaeridium tubiferum (Ehrenberg) . A specimen from the London 

 Clay. Left, apical view showing the archaeopyle, precingular and cingular processes, 

 and a single sulcal process ; right, antapical view (by transparency) showing the pre- 

 cingular, sulcal, posterior intercalary and antapical processes. x c. 1000. 



