ioo MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Dimensions. Holotype : length of central body 36-5[x, breadth of central body 

 23-5^, length of processes 14-10,(1, number of processes approximately 65. Range : 

 length of central body 30-4410., breadth of central body 2i-24[x, length of processes 

 12-igfx. Number of specimens measured, 4. 



Description. The periphragm of the central body is granular and bears a number 

 of quite large tubercles. The periphragm of the processes is smooth. The proces- 

 ses have relatively broad bases, up to 4[x wide and taper distally being i-i-5[jl wide 

 for most of their length. At their distal extremities they widen slightly and have a 

 somewhat serrate margin. The processes are arranged in circular series around the 

 central body, indicating a reflected dinoflagellate tabulation. Two or, more rarely, 

 three processes are present for each plate area. 



T. regulate has been recorded from the London Clay of Whitecliff Bay and of 

 Enborne, Berkshire. 



Remarks. The distinctive elongate nature of the central body of T. regulare is 

 typical of this genus. T. regulare differs from T. variecalamum in having more 

 processes, and from T. isocalamus comb. nov. in having slenderer processes with 

 more complex terminations. 



OTHER SPECIES 



The following species are here attributed to the genus Tanyosphaeridium on the 

 basis of the shape of the central body and the form of the processes : 



Tanyosphaeridium ellipticum (Cookson 1965). Upper Eocene ; Australia. 

 Tanyosphaeridium isocalamus (Deflandre & Cookson 1955). Lower Cretaceous ; 

 Australia. 



Genus HOMOTRYBLIUM nov. 



Derivation of name. Greek, homos, same or similar ; tryblion, cup or bowl — 

 with reference to the formation of two, almost equal, hemispheres after rupture of the 

 cyst. 



Diagnosis. Sub-spherical chorate cyst with central body composed of thin 

 endophragm and surrounding periphragm which gives rise to processes. Processes 

 intratabular, cylindrical to tubiform, open distally, reflecting a tabulation of 3', 6", 

 6c, 6'", ip, 1"" and 1 to 5s. Processes not in communication with cavity of the 

 central body. Archaeopyle epitractal, suture running just above cingulum processes. 



Type species. Homotryblium tenuispinum sp. nov. Eocene ; England. 



Description. The genus Homotryblium is unusual in possessing an epitractal 

 archaeopyle which has a compound operculum composed of the apical and precingu- 

 lar plate series. It is an easily recognizable genus because of the nature of this 

 archaeopyle and the possession of only 3 apical processes. Dinoflagellate cysts 

 possessing epitractal archaeopyles are rare. Rhaetogonaulax gen. nov. and Dichado- 

 gonyaulax gen. nov. are both described by Sarjeant, in a later section, as possessing 

 epitractal archaeopyles ; 



