MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 79 



Diagnosis. Chorate, sub-spherical to ovoidal cyst bearing two types of processes. 

 One with broad open infundibular processes, often fenestrate distally ; the other 

 with smaller slender processes, closed, with simple or bifurcate extremities. Apical 

 archaeopyle present. 



Holotype. B.M.(N.H.) slide ¥.51743(1). 155 feet above base of London Clay ; 

 Whitecliff Bay, Isle of Wight. 



Dimensions. Holotype : diameter of central body 42-5 by 43^ ; length of 

 infundibular processes 24-29^ ; length of slender processes up to ig\i. Range : 

 diameter of central body 42-57^ ; length of infundibular processes 24-401X ; width 

 of infundibular processes up to g\i proximally, 34^ distally ; length of slender 

 processes up to 27[x ; number of specimens measured, 4. 



Description. This species has very distinctive infundibular processes with an 

 irregular digitate margin, each branch being further split up into evexate, bulbous 

 or bifid secae. The secae and/or the digitae may be inter-connected, thus giving rise 

 to a fenestrate structure. The tabulation reflected by the infundibular processes is 

 4', 6", 5'", ip, 1"". In addition there are some very slender processes, less than I[jl 

 wide, restricted to the cingulum and sulcal zones. These processes are always 

 closed, and are either acuminate or irregularly bifurcate. The exact number is 

 variable and difficult to determine. 



P. pannosum has only been recorded from the Ypresian of Britain. 



Remarks. This species differs from Oligosphaeridium palcherrimum (Deflandre 

 & Cookson) from the Australian Lower Cretaceous, in the nature of the fenestrate, 

 infundibular processes. The slender cingular and sulcal processes are a character- 

 istic feature. 



OTHER SPECIES 



Perisseiasphaeridium eisenacki sp. nov. The specimens described and figured by 

 Eisenack (1958 : 402, pi. 26, figs. 1, 2) as Hystrichosphaeridium anthophorum Cookson 

 & Eisenack from the Lower Cretaceous of Germany are here considered to belong to 

 the genus Perisseiasphaeridium nov. and have been renamed accordingly. 



Evitt (1961) pointed out that Eisenack's specimens possess definite fine cingular 

 processes whereas the type material from the Upper Jurassic of Australia and Papua 

 has a barren cingular zone and has been attributed to the genus Oligosphaeridium 

 (see p. 77). 



Genus LITOSPHAERIDIUM nov. 



Derivation of name. Greek, litos, plain or simple ; sphaera, ball — with 

 reference to the simple arrangement of the processes on the surface of the central body. 



Diagnosis. Chorate cysts with spherical to sub-spherical central body composed 

 of two membranes. Processes few in number (only one per plate area), hollow and 

 open distally except for sulcal processes which may be closed. Reflected tabulation 

 3', 6", 5'", ip, 1"", with variable number of sulcal processes. Archaeopyle apical. 



