92 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



greater than one, not normally more than four. Processes hollow, open or closed 

 distally, process cavity not connecting with interior of central body. Archaeopyle, 

 when present, apical with angular margin. 



Type species. Polysphaeridium subtilum sp. nov. Eocene (Ypresian) ; England. 



Remarks. The processes are typically fairly short and expand distally, termina- 

 ting by a serrate or spinous margin. The number of processes present makes it 

 extremely difficult for the reflected tabulation to be determined, but is undoubtedly 

 basically the same as that possessed by Hystrichosphaeridium tubiferum (Ehr). 



As this genus now stands it contains a large number of species having in common 

 an archaeopyle and a large number of hollow processes, but otherwise unrelated. 

 At some future date, fuller knowledge of this genus will undoubtedly necessitate its 

 subdivision, perhaps on the basis of process form. 



Polysphaeridium subtile sp. nov. 

 PI. ii, fig. i 



Derivation of name. Latin, subtilus, thin or slender — with reference to the 

 slenderness of the processes. 



Diagnosis. Central body, sub-spherical to sub-rectangular, with wall composed 

 of extremely thin smooth endophragm and thin granular periphragm. Processes 

 not in connection with interior, slender, open distally, with serrate margin. Average 

 length of processes, one-fifth to one-third the diameter of central body. Number of 

 processes greater than 60. 



Holotype. B.M.(N.H.) slide ¥.51752(1). 178 feet above base of London Clay; 

 Sheppey, Kent. 



Dimensions. Holotype : diameter of central body 47-48^, length of processes 

 io-i6jx. Range : diameter of central body 3i-5-50jx, length of processes 6-i6jx. 

 Number of specimens measured, 5. 



Description. Simple, slender, tubiform processes possessing a usually serrate 

 distal margin. The latter is occasionally entire, and may be circular or oval. 

 The width of the processes never exceeds 2\i. The processes do not appear to be 

 arranged in any regular pattern. 



Remarks. P. subtile sp. nov. is distinguished from all other species by the 

 number and form of the open processes present. 



Polysphaeridium pastielsi sp. nov. 

 PI. 4, fig. 10 



1948. Hystrichosphaeridium pseudhystrichodinium (Deflandre) ; Pastiels : 43, 3, figs. 12-15. 



Derivation of name. In honour of Andre" Pastiels, who made pioneer studies in 

 the Eocene microplankton of Belgium. 



