234 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Genus THALASSIPHORA Eisenack & Gocht i960 : 51 



Emended diagnosis. Pterate cysts with spherical to ellipsoidal central body, 

 smooth or more commonly granular. Periphragm in form of helmet-shaped 

 " lamellar wing " and in contact with endophragm only on dorsal surface of central 

 body. Archaeopyle precingular. 



Type species. Bion pelagicum Eisenack 1938. Oligocene ; East Prussia. 



Discussion. Thalassiphora Eisenack & Gocht is now considered to be referable to 

 the Dinophyceae (Downie, Evitt & Sarjeant 1963) on account of the frequently 

 occurring archaeopyle. As Alberti (1961) noted, the structure of Thalassiphora 

 resembles that of Pterospermopsis superficially but detailed examination shows that 

 in the former, the periphragm is attached to the endophragm on one surface of the 

 central body only, the dorsal surface. The periphragm extends considerably 

 beyond the central body as a wing lamella, in shape resembling an inverted basin 

 with the margin often turned over. The position of the archaeopyle is constantly 

 dorsal anterior, the keel when present, is posterior. If an archaeopyle is present, it 

 occurs in the periphragm and endophragm. Free opercula are often encountered in 

 sample. 



In Pterospermopsis, the wing lamella is in contact with the central body along an 

 " equatorial zone " only. Species of Pterospermopsis do not possess an archaeopyle. 



Thalassiphora pelagica (Eisenack) 

 PI. 26, fig. 7 



1938. Bion pelagicum. Eisenack : 187. 



1954. Pterospermopsis pelagica (Eisenack) Eisenack : 71, pi. 12, figs. 17, 18. 



i960. Thalassiphora pelagica (Eisenack) Eisenack & Gocht : 513, text-figs. 1-3. 



1961. Pterospermopsis pelagica (Eisenack) ; Gerlach : 209, pi. 28, fig. 15. 



1963. Thalassiphora pelagica (Eisenack) ; Gerlach : 50, pi. 3, fig. 3. 



1963. Thalassiphora pelagica (Eisenack) ; Brosius : 50, pi. 3, fig. 3. 



Discussion. T. pelagica is characterized by the presence of a keel on the peri- 

 phragm. In the London Clay, orientation of the specimens has been guided by the 

 position of the archaeopyle, which forms by the loss of a single precingular plate. 

 The archaeopyle is anterior in position, the keel always being posterior with respect 

 to this. The face of the central body containing the archaeopyle is dorsal. The 

 archaeopyle is present in the periphragm and endophragm and has a convex tri- 

 angular outline. Specimens with the operculum lying within the central body have 

 been observed. The ovoidal central body has a wall up to i-$\i thick. The thick 

 periphragm is fibrous with a reticulate ornamentation ; it can be perforate. In 

 size, the London Clay forms show close agreement with the type material. 



Dimensions. Range observed in London Clay ; overall diameter 170-220^ ; 

 central body 85-107^ ; archaeopyle 30-42^ ; keel I3[x. Number of specimens 

 measured, 6. 



Occurrence. Eocene, London Clay ; Sheppey, Kent, and from the Upper 

 Eocene to Middle Miocene of Germany. 



