MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 87 



Cordosphaeridium cracenospinosum sp. nov. 

 PI- 3. %• 4 



Derivation of name. Latin, cracens, graceful or slender; spinosus, thorny — 

 with reference to the appearance of the processes. 



Diagnosis. Sub-spherical to polygonal central body with wall composed of 

 endophragm and periphragm, up to i'5(x in thickness. Endophragm very thin. 

 Archaeopyle apical haplotabular ; processes slender, buccinate, erect, or curved, 

 solid or with fine central tubule or hollow, the last two types open distally. Distal 

 margins foliate, bifurcate or digitate. Processes one per plate area. 



Holotype. B.M.(N.H.) slide ¥.51748(1). Metropolitan Water Board Borehole 

 No. 39 at 170 feet depth, London Clay ; Enborne, Berkshire. 



Dimensions. Holotype : diameter of central body 122 by I25fi, length of 

 processes up to 66[x. Range : diameter of central body 75-125^, length of processes 

 17-76^. Number of specimens measured, 5. 



Description. The large central body of C. cracenospinosum sp. nov. has a finely 

 reticulate surface. In overall outline this species appears to be sub-spherical, but 

 closer examination shows that often it tends to be polygonal. The processes are 

 generally single, rarely branched, and in length often exceed half the diameter of the 

 central body. The periphragm often forms a ridge on the surface of the central body 

 where a process arises. Occasionally very fine, shorter processes are present. 

 Many of the processes are solid, others having a narrow central cavity throughout 

 their length or being hollow tubiform. Distally there is considerable variation, 

 digitate endings being the commonest. 



This species of Cordosphaeridium has been observed in samples throughout the 

 London Clay of England. 



Remarks. C. cracenospinosum differs from C. gracilis by its thinner wall, which is 

 only very faintly fibrous, the shape of the central body and the occasional presence 

 of smaller, very fine, processes. It is unusual in having solid and hollow processes, the 

 former however predominate. Process endings comparable to those in Baltisphae- 

 ridium pectiniforme Gerlack (1961) from the Oligocene of Germany, are occasionally 

 encountered. 



Cordosphaeridium exilimurum sp. nov. 

 PI. 11, fig. 2 



Derivation of name. Latin, exilis, thin or meagre ; murus wall — with refer- 

 ence to the rather thin wall of the central body. 



Diagnosis. Ovoidal central body, composed of thin, smooth or slightly granular 

 endophragm with fine fibrils of periphragm running over surface. Processes 

 tubiform or buccinate, of variable width, and rarely latispinous, distally open with 

 serrate or undulose margin. Wall of processes thin and often fenestrate. 



