180 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Description. The tabulation of H. unispinum is well shown in two specimens 

 from the London Clay. There are four apical plates, i' being smaller than the other 

 three. The apical processes are tapering, open distally with a serrate or undulose 

 margin. Five of the precingular plates and processes are equal in size, the other, 

 plate 6", is smaller, more closely approaching the anterior sulcal plate. The equa- 

 torial plates each possess one process. The postcingulars are as in H. eisenacki, with 

 plate i'" being reduced compared to the other four. The process of the posterior 

 intercalary plate is small and the antapical plate is marked by a longer than average 

 process. 



The broad processes all appear open distally and may have small tubular branches. 

 The equatorial processes are broader than in other species of Hystrichokolpoma and 

 are tubiform or buccinate with serrate or undulose distal margins. A few have 

 perforations in the wall, irregular in position. The processes may occasionally be 

 branched. The posterior sulcal processes are slender and short with digitate 

 endings which may be open or closed. 



Occurrence. London Clay ; Whitecliff and Enborne. 



Remarks. The presence of one process only on each cingular plate readily 

 distinguishes H. unispinum from other species of Hystrichokolpoma. 



Hystrichokolpoma rigaudae Deflandre & Cookson 

 PL 17, fig. 4 



1954 

 1955 

 1959 

 1961 

 1962 



1963 

 1964 



Hystrichokolpoma rigaudae Deflandre & Cookson, text-fig. 15 (n.n.). 

 Hystrichokolpoma rigaudae Deflandre & Cookson : 279, pi. 6, figs. 6-10 ; text-fig. 42. 

 Hystrichokolpoma rigaudae Deflandre & Cookson ; Maier : 311, pi. 31, fig. 2. 

 Hystrichokolpoma rigaudae Deflandre & Cookson ; Gerlach : 183, pi. 27, figs. 8, 9. 

 Hystrichokolpoma rigaudae Deflandre & Cookson ; Rossignol : 134. 

 Hystrichocolpoma rigaudae Deflandre & Cookson ; Brosius : 43, pi. 2, fig. 6. 

 Hystrichokolpoma rigaudae Deflandre & Cookson ; Rossignol : 89, pi. 2, fig. 5, pi. 3, 

 fig. 8. 



Discussion. Specimens of H. rigaudae from the London Clay possess a tabulation 

 of 6", 6c, 5'", ip, 1"" and 6a. Plates 6" and 1'" and their attendant processes are 

 reduced whilst the anterior sulcal plate is the largest of the six. A few of the broad 

 processes and some of the girdle ones are open distally. There seems to be a degree 

 of variability in the number of cingular processes, some plates having only one ; 

 generally however there are two processes on each cingular plate. 



Dimensions. Observed range in London Clay : diameter of central body 35-481X. 

 Length of broad processes up to 3ijx. Length of antapical process up to 39(1.. 

 Number of specimens measured, 3. 



Occurrence. London Clay ; Whitecliff, Enborne and Sheppey. H. rigaudae 

 has also been recorded from the Eocene and Miocene (or older) of Australia (Deflandre 

 & Cookson 1954, 1955), the Middle Oligocene-Middle Miocene (Gerlach 1961), the 

 Upper Oligocene (Brosuis 1963) and the Middle Miocene of Germany (Maier 1959) 

 and the Pleistocene of Israel (Rossignol 1962, 1964). 



