MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 221 



Holotype. B.M.(N.H.) slide ¥.51959(2), London Clay ; Sheppey, Kent, sample 

 Sh 1. 



Dimensions. Holotype : diameter of central body 42 by 43[x. Length of 

 processes up to 26^. Observed range : diameter of central body 35-44^. Length 

 of processes 7-24^. Number of specimens measured, 8. 



Description. The central body has a thin wall, smooth or slightly granular, 

 comprising the endophragm. When an archaeopyle is present, the zigzag margin 

 sharply delimits six rectangular plates, each bearing a process. The sulcal notch is 

 usually clearly discernible. The process denoting the posterior intercalary plate is 

 more commonly absent. 



The fibrous processes rarely exceed two thirds of the diameter of the central body 

 in length. They are generally simple and few in number, this being governed by the 

 number of plates present. The size of the process may be a reflection of its position 

 on the central body, the postcingulars often being smaller than the precingular 

 processes. The cingulum appears to be slightly helicoidal. 



The meshwork of the reticulate membrane is variable in size within an individual : 

 it is finer nearer the process. The membrane is not unlike a closely woven net in 

 structure, with numerous fine interconnecting orthogonal threads. The membrane 

 may be restricted to distinct zones as in the figured specimen. There the processes 

 of each series of plates are distally united and have few interconnecting links with 

 processes of other plate series. This can be useful in orientation since it clarifies the 

 position of the different plate series, and particularly the pre- and postcingulars. 



Two variants of M. reticulata are recognized based on the arrangement of the 

 processes. These are : 



Var. a. Reflected tabulation as in specific diagnosis but with no cingular processes. 

 Sulcal processes may be present. This is the commoner of the two variants. 



Var. b. Tabulation as in specific diagnosis with cingular processes present, there 

 being four in number. 



Variability in M. reticulata also extends to the number of apical processes, some 

 specimens having one, others having four. However since it is impossible to dis- 

 tinguish them when the archaeopyle is developed, the two forms are treated together 

 as parts of the same species. 



Occurrence. London Clay ; Whitechff, Enborne and Sheppey. 



Remarks. Comparison of M. reticulata with other described forms suggests close 

 affinity with Membranilarnax sp. O. Wetzel (1936), from the Upper Eocene of Ger- 

 many. Reissinger (1950) figured a similar form which he simply termed a " hystri- 

 chosphere ", this also being from the Eocene of Germany. Unfortunately neither of 

 these specimens have been preserved, and only Wetzel attempted a description, 

 brief in the extreme, stating that the form was a spiny sphere within an outer shell ; 

 a description too succinct to be useful. 



Eisenack (19546) figured a specimen which he compared with Membranilarnax 

 sp., O. Wetzel (1936). Eisenack (1959c) however inferred that the outer membrane 



