MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 165 



Genus EXOCHOSPHAERIDIUM nov. 



Derivation of name. Greek, exochos, projecting or prominent ; sphaera, 

 ball — with reference to the distinctive nature of the apical process. 



Diagnosis. Subspherical chorate cysts bearing numerous, commonly simple, 

 closed processes. Apical process larger than normal processes and irregularly 

 branched. Archaeopyle precingular. 



Type species. Exochosphaeridium phragmites sp. nov. Upper Cretaceous (Ceno- 

 manian) ; England. 



Remarks. The processes are commonly acuminate, often joining proximally, but 

 may be branched. Only rarely can any alignment of these processes be observed. 

 The apical process makes orientation easy and indicates that the archaeopyle is 

 precingular. Detached archaeopyle plates have been found and these show the 

 characteristic precingular shape. 



Exochosphaeridium phragmites sp. nov. 



PI. 2, figs. 8 — 10 



Derivation of name. Latin, phragmites, a reed — with reference to the reed-like 

 shape of the processes of this species. 



Diagnosis. Central body subspherical to oval, possessing a pitted surface and 

 bearing numerous acuminate processes. Processes solid or fibrous, broad-based, 

 bases of adjacent processes often confluent. Distinctively branched apical process 

 present and commonly a precingular archaeopyle. 



Holotype. Geol. Surv. Colin. PF.3035(3). Lower Chalk, H.M. Geological 

 Survey Borehole, Fetcham Mill, Surrey at 840 ft. depth. Upper Cretaceous (Ceno- 

 manian). 



Paratype. Geol. Surv. Colin. PF.3043(i). H.M. Geological Survey Borehole, 

 Fetcham Mill, Surrey at 810 feet depth. Upper Cretaceous (Cenomanian). 



Dimensions. Holotype : diameter of central body 49 by $6\l, length of processes 

 up to 22(x. Paratype : diameter of central body 33 by 36^, length of processes up to 



22(X. 



Description. The fibrous processes may occasionally be slightly perforate. 

 Division of a process into two often takes place medially, and more rarely distally. 

 The processes may terminate distally in a point or may be blunted. The arrangement 

 of the processes usually appears to be haphazard and in only one specimen, the para- 

 type, can any alignment be observed. In this specimen a definite alignment can be 

 seen on both sides of the cingulum running parallel to this structure. An apical 

 process and an archaeopyle are present, thus making orientation easy. The apical 

 process is very distinctive, being foliate in shape ; it is situated near the edge of the 

 archaeopyle. The position of this process indicates that it must be an apical process 

 and that the archaeopyle is precingular. E. phragmites occurs throughout the 

 Cenomanian of England. 



