MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 93 



Diagnosis. Ovoidal central body with smooth or granular surface. Apical 

 archaeopyle with zig-zag margin. Processes numerous, all of one type ; simple 

 open, tapering distally to narrow neck before spreading slightly to an opening with 

 entire or serrate margin. Processes sometimes united proximally, slightly fibrous. 



Holotype. B.M.(N.H.) slide V. 51753(1). 99 feet above base of London Clay ; 

 Sheppey, Kent. 



Dimensions. Holotype : diameter of central body 37-43^, length of processes 

 10-14^. Range : diameter of central body 31-43^, length of processes up to ig\i. 

 Number of specimens measured, 5. 



Description. The processes broaden proximally, often quite considerably, and 

 may measure up to 9 [i. in width but the latter is very variable. An apical archaeo- 

 pyle is present. 



Remarks. The Ypresian forms illustrated by Pastiels (1948) as Hystricho- 

 sphaeridium pseudohystrichodinium strongly resemble the London Clay forms. The 

 holotype of Deflandre has long supple processes, thinning regularly towards the 

 extremity which is simple, or fluted, or with a very small fork. P. pastielsi is 

 therefore a separate species which superficially resembles H. pseudohystrichodinium. 



IP. asperum (Maier) is described as having an oval granular central body. The 

 processes are open, tapering, terminating distally in 2-3 spines. This distal margin 

 is not unlike the denticulate lip sometimes observed in P. pastielsi. The two species 

 may even be conspecific. 



P. simplex (White) as illustrated by Gerlach (1961) and by Brosius (1963) are both 

 probably conspecific with P. pastielsi. They differ from White's holotype which 

 possesses longer and fewer processes with greatly expanded distal openings. 



Polysphaeridium pumilum sp. nov. 

 PI. 7, figs. 3, 4 



?i955. Hystrichosphaeridium recurvation (White) ; Deflandre & Cookson : 269, pi. 1. fig. 12. 



Derivation of name. Latin, pumilus, dwarfish or little — with reference to the 

 small size of this species. 



Diagnosis. Subspherical central body having numerous small open tubular 

 processes. Processes terminating distally in a slightly recurved more or less entire 

 margin. Length of processes less than half that of the maximum body diameter. 



Holotype. Geol. Surv. Colin, slide PF.3037(i). Lower Chalk, H.M. Geological 

 Survey Borehole, Fetcham Mill, Surrey at 750 feet depth. Upper Cretaceous 

 (Cenomanian). 



Dimensions. Holotype : overall diameter 40 by 34^, diameter of central body 

 25 by I9[jl, length of processes 8-iO[i.. Number of processes c.44. Range : overall 

 diameter 30-40^, diameter of central body 17-25^, length of processes 7-iojj., 



