MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 51 



Achomosphaera sagena sp. nov. 

 PI. 2, figs. 1, 2 



Derivation of name. Latin, sagena, fish-net — with reference to the distinctive 

 net-like pattern on the surface of the central body. 



Diagnosis. Wall of central body extremely thick, apparently composed of 

 columar elements, surface coarsely reticulate. Closed gonal processes hollow, 

 trifurcate with bifurcate extremities, and commonly possessing reticulate bases. 

 Processes sometimes branched, branding mainly confined to cingular zone. Charac- 

 teristically shaped precingular archaeopyle often present. 



Holotype. Geol. Surv. Colin, slide PF.304i(i). Lower Chalk, H.M. Geological 

 Survey Borehole, Fetcham Mill, Surrey, at 650 feet depth. Upper Cretaceous 

 (Cenomanian). 



Dimensions. Holotype : diameter of central body 48^, length of processes up to 

 20pL. Range : diameter of central body 35-59^, length of processes up to 28[x. 

 Number of specimens measured, 8. 



Description. The wall of the central body, which may be up to 5\l thick, is 

 composed of elongate cellular elements lying perpendicular to the surface. The 

 surface reticulation is coarse and may be up to i*5[x across (PI. 2, fig. 2). 



A . sagena sp. nov. has only been recorded from the Cenomanian of England. 



Remarks. A. sagena is similar to H. crassipellis (Deflandre & Cookson) in that 

 both possess thick walls of an identical nature with a reticulate surface. However 

 sutural crests are absent in A . sagena. 



Achomosphaera neptuni (Eisenack) 

 PL 3, fig. 7 ; PL 9, fig. 11 



1958. Baltisphaeridium neptuni Eisenack : 399, pi. 26, figs. 7, 8 ; text-fig. 8. 



1959. Baltisphaeridium neptuni Eisenack ; Gocht : 73, pi. 4, fig. 14 



Description. A. neptuni possesses a central body with a reticulate or sometimes 

 slightly fibrous surface. The processes are gonal in position, taeniate or taeniate- 

 triangular and may be bifurcate or trifurcate. In the cingular zone the processes 

 are branched. The processes are fibrous, the fibres sometimes radiating from the 

 bases of the processes and these may be slightly thickened along the reflected plate 

 boundaries joining the processes. A precingular archaeopyle is often present. 



Material (figured). B.M.(N.H.) slides V.51716-17. Speeton Clay, Shell West 

 Heslerton boring at 103-25 metres depth, West Heslerton, Yorkshire. Lower 

 Cretaceous (Middle Hauterivian). 



Dimensions. V.51716, diameter of central body 61 by 66\i, length of processes 

 up to 28(x. V.51717, diameter of central body 47 by 52^, length of processes up to 



2I(Jl. 



