46 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Description. Distally the processes may be simple, bi- tri- or tetrafurcate. 

 When the processes are furcate, the furcations are digitate and always erect. Each 

 taeniate furcation may be bifid or have a serrate or entire distal margin. Some of 

 the processes are fenestrate proximally. 



Remarks. The concentration of granules along the plate boundaries, the spheri- 

 cal shape of the central body and the short erect processes differentiate H. monilis 

 sp. nov. from all other described species of Hystrichosphaera. 



Hystrichosphaera sp. 



PL 9, fig. 9 



Description. A type of Hystrichosphaera with coarsely reticulate periphragm 

 forming crests and processes, as well as surface of central body. The crests are well 

 developed and bear small protruberances or stunted processes which terminate 

 bluntly or are bifid. Gonal and a small number of sutural processes are present. 



Material (figured). B.M.(N.H.), slide ¥.51724(1). Speeton Clay, Shell West 

 Heslerton Borehole at 42-5 metres depth, West Heslerton, Yorkshire. Lower 

 Cretaceous (Lower Barremian). 



Dimensions. Figured specimen : diameter of central body 39 by 39^, length of 

 processes up to iijx. Second specimen : diameter of central body 43 by 5i[x, 

 length of processes up to I0[z. 



Remarks. This form has been encountered only rarely at one horizon, and it is 

 characterized by its coarsely reticulate periphragm. It differs from H. ramosa in 

 the reduced state of the processes and from H. cingulata var. reticulata in the presence 

 of the latter. Since only two specimens were available for study and the presence of 

 the typical Hystrichosphaera reflected tabulation was not verified. No specific name 

 has been given. 



Genus ACHOMOSPHAERA Evitt 1963 : 163 



Diagnosis. Test consisting of spherical to ellipsoidal central body with pre- 

 cingular archaeopyle and furcate, spine-like processes like those of Hystrichosphaera 

 in both structure and distribution, but without sutural ridges or septa connecting 

 their bases as in that genus. Tips of processes not connected. Wall two-layered ; 

 layers typically in close contact between bases of processes. 



Type species. Hystrichosphaeridium ramuliferum Deflandre 1937. 



Remarks. One of the authors (R. J. D.) was allowed, by kind permission of 

 Professor Deflandre, to examine the type material. One paratype, figured by 

 Deflandre (1937, pi. 14, fig. 6), under high magnification was seen to possess very 

 faint lines on the surface of the central body delimiting the plate boundaries. Such 

 lines were not observed on the holotype but this was probably due to the obscuring 

 nature of particles within the flint. These faint lines, slight thickenings of the 

 periphragm and comparable to the sutural crests of Hystrichosphaera, have been 

 observed in a number of chemically prepared specimens belonging to this genus. 



