76 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Material (figured). B.M.(N.H.) slide ¥.51742(1). Metropolitan Water Board 

 Borehole No. 11 at 63 feet depth, London Clay ; Enborne, Berkshire. 



Remarks. The processes of the London Clay specimens indicate a reflected 

 tabulation of 4', 6", 6'", ip, 1"". The apical processes are relatively small and an 

 apical archaeopyle is usually present. The fenestrate appearance of the processes in 

 this species are extremely distinctive. The London Clay forms may be derived. 



Dimensions. London Clay forms : diameter of central body 30-48^, length of 

 processes 17-40(1. Number of specimens measured, 3. 



Oligosphaeridium prolixispinosum sp. nov. 

 PL 8, figs. 2, 3 



Derivation of name. Latin, prolixus, stretched out long and spina, thorny — 

 with reference to the filiform spines at the distal extremities of the processes. 



Diagnosis. Elongate central body bearing few open tubular processes. Proces- 

 ses terminating distally in a number of long thread-like spines. 



Holotype. Geol. Surv. Colin, slide PF.3036(i). Lower Chalk, H.M. Geological 

 Survey Borehole, Fetcham Mill, Surrey, at 840 feet depth. Upper Cretaceous 

 (Cenomanian). 



Dimensions. Holotype : overall width 64jx, diameter of central body 40 by 20\i, 

 length of processes i8-24[x, number of processes 17. Range : Length of central body, 

 33-43[x, width 20-2gjx, length of processes i8-29[x. Number of specimens measured, 



8. 



Description. The periphragm of the central body and of the processes is smooth. 

 The processes are thin- walled and have a fairly broad base up to 8[i, with character- 

 istic basal wrinkles. There are noticeable circles beneath the processes where only 

 endophragm is present. The processes are about 3[A in width for most of their 

 length tapering to 2(jl distally before giving rise to the filiform spines. The spines are 

 extremely delicate, 5 to 8 in number, and up to I5[x in length. The number of 

 processes varies from 16 to 18, the variation probably due to the number of sulcal 

 processes present. In all specimens there is an obvious apical archaeopyle. Sur- 

 rounding the archaeopyle are 6 precingular processes, then a diastema before the 

 postcingular processes are reached. This diastema marks the position of the 

 cingulum. The processes on the hypotract are difficult to assign to any dinoflagellate 

 tabulation. There is no obvious antapical process, in fact 3 antapical processes 

 usually seem to be present. The reflected tabulation appears to be (4'), 6", 6'", 

 3"" and 1-3S. 



0. prolixispinosum sp. nov. is a rare species occurring throughout the Cenomanian 

 of Surrey. 



