MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 227 



Remarks. C. ordinum differs from C. divaricum in the regular arrangement of the 

 processes in linear complexes and the larger overall size. It is included within the 

 genus Cyclonephelium on account of the absence of processes from the mid-dorsal and 

 ventral surface apart from those on the margin of the archaeopyle. 



Cyclonephelium pastielsi Deflandre & Cookson 

 PI. 25, fig. 2 



1948. Membranilamax cf. liradiscoides Pastiels : 47, pi. 5, fig. 15. 

 1955. Cyclonephelium pastielsi Deflandre & Cookson : 285. 



Discussion. Specimens of C. pastielsi from the London Clay almost invariably 

 possess an archaeopyle, apical in position and with a zigzag margin. The prominent 

 sulcal notch lies to the right of the mid-ventral line. Only rarely is a complete 

 individual, with the apex in place, encountered. The numerous, solid taeniate 

 processes are complexly united along their length and distally. They are frequently 

 arranged in linear complexes. Proximally the processes arise singly or in groups of 

 twos or threes. Distally the interconnecting trabeculae may be perforate, up to 5 to 

 6[x in width. Unconnected short, slender, acuminate or bifid spines often arise from 

 the trabeculae. Occasional simple acuminate processes occur on the central body. 



Pastiels figured a specimen of C. pastielsi (as Membranilamax cf . liradiscoides, pi. 5, 

 fig. 15) showing an absence of processes round the mid-ventral and mid-dorsal mar- 

 gins of the archaeopyle. In the majority of the London Clay forms the processes are 

 rather more numerous on the ambitus and also completely surround the archaeopyle 

 margin. Those processes round the archaeopyle are more complex in structure than 

 the ambital processes, and are aligned in rows parallel to the margin of the archeo- 

 pyle. It is probable that each precingular and postcingular plate has processes to a 

 greater or lesser degree. 



Specimens of C. pastielsi can have two antapical protuberances, one more strongly 

 developed than the other. When these are present the outline of the central body 

 is closely comparable to that of Areoligera. The size of the London Clay forms 

 usually exceeds that of the type material. Transitional forms to C. exuberans are 

 not uncommon. 



Dimensions. Observed range in London Clay : diameter of central body 

 43-90^, length of processes 12-38^. Number of specimens measured, 25. 



Occurrence. Common at all horizons of the London Clay. The forms having 

 the larger central bodies are commonest in WC.14, 26 and E. 11/88. C. pastielsi 

 has also been recorded from the Ypresian of Belgium (Pastiels 1948). 



Genus AREOLIGERA Lejeune-Carpentier 1938 : 164 



Emended diagnosis. Chorate cysts with hemispheral central body, convex 

 dorsal side and flat or depressed ventral side. Processes intratabular, on dorsal 

 surface arranged in soleate or annular complexes, on ventral surface in linear or 



