MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 75 



Holotype. B.M.(N.H.) slide ¥.51709(1). Speeton Clay, Shell West Heslerton 

 Borehole No. 1, Yorkshire, at 99-25 metres depth. Lower Cretaceous (Middle 

 Hauterivian) . 



Dimensions. Holotype : overall length 107- 5[x, breadth n6fx, central body 

 length 46-5[x (apical region lacking), breadth 52^. 



Paratype. B.M.(N.H.) slide ¥.51709(3) : overall length 124-5^ breadth n6pi, 

 central body length 46-5[i (apical region lacking), breadth 46-5^1. Range ; overall 

 lengths 107-5-130^. (7 specimens). 



Description. The periphragm on the surface of the central body is moderately 

 thick and pitted to a varying extent. The pits, passing through the periphragm and 

 exposing the endophragm, vary greatly in size, being as much as ly. across. The 

 processes arise from root-like bases and terminate distally in 4-6 simple spines. 

 The processes do not connect with the interior of the central body. Rarely the 

 processes possess small, often rectangular, areas where the periphragm is extremely 

 thin, formerly described as perforations of the processes. 



An apical archaeopyle is usually present, the resulting specimens possessing 14 

 processes. The inferred reflected tabulation is typical of the genus. 



The species has only been recorded from the Middle Hauterivian. 



Remarks. 0. vasiformum is very similar to 0. complex from the Barremian and is 

 clearly related to it. The most important distinguishing features is the presence of 

 the pitted periphragm on the surface of the central body in 0. vasiformum. The 

 original tabulation of this species is tentatively reconstructed in Text-fig. 1. 



0. reticulatum sp. nov. is another related form, but its processes are usually 

 noticeably shorter and stouter with a much more complex distal margin. 



Oligosphaeridium macrotubulum (Neale & Sarjeant) 

 PI. 9, fig. 4 



1962. Hystvichosphaeridium macrotubulum Neale & Sarjeant : 452, pi. 20, fig. 7 ; text-fig. 8a. 



Remarks. A re-examination of the holotype has shown that the periphragm of 

 the central body is pitted in exactly the same manner as in 0. vasiformum, albeit to 

 a lesser extent. The processes have basically the same structure as in the latter 

 species, but are perhaps slightly stouter than is typical. The holotype does not 

 possess an archaeopyle, there being 18 processes present, indicating the reflected 

 tabulation typical of this genus. As only the holotype of 0. macrotubulum was 

 examined the range of variation cannot be ascertained, but it is probable that this 

 species is synonymous with 0. vasiformum. 



Oligosphaeridium pulcherrimum (Deflandre & Cookson) 

 PI. 10, fig. 9 ; PI. 11, fig. 5 



1955. Hystvichosphaeridium pulcherrimum Deflandre & Cookson : 270, pi. 1, fig. 8 text-figs, 21, 



22. 

 1955. Hystvichosphaeridium pulcherrimum Deflandre & Cookson ; Valensi : 592, pi. 4, fig. 1. 



