66 MESOZOIC AND CAINOZOIC DINOFLAGELL ATE CYSTS 



Hystrichosphaeridium mantelli sp. nov. 

 PI. 6, fig. 6 



Derivation of name. In honour of the geologist and microscopist Gideon 

 Algernon Mantell, who did much pioneer work in interpreting the structure of 

 Upper Cretaceous microplankton during the mid-nineteenth century. 



Diagnosis. Spherical to sub-spherical central body composed of thin endo- 

 phragm and granular or finely reticulate periphragm. Periphragm of processes 

 slightly fibrous. Processes predominantly simple, tubiform, buccinate or bulbose, 

 open distally with digitate or foliate margin. Occasionally narrow, solid, closed 

 processes occur. Number of processes 26 or less, one process per plate area. Apical 

 archaeopyle usually present. 



Holotype. Geol. Surv. Colin, slide PF. 3032(1). Lower Chalk, H.M. Geological 

 Survey Borehole, Fetcham Mill, Surrey, at 840 feet depth. Upper Cretaceous 

 (Cenomanian). 



Dimensions. Holotype : diameter of central body 41 by 42^, length of processes 

 13-2 1 [i, number of processes 25. Range : diameter of central body 36-45^, length 

 of processes i3-26[x. Number of specimens measured, 6. 



Description. At first sight the periphragm of the central body appears to be 

 fairly heavily granular, but on closer examination the granules apparently result 

 from a fine reticulation. The processes are erect and extremely variable in form, 

 being tubiform, buccinate or even bulbose, usually open but occasionally closed, and 

 simple or branched distally. The closed processes are extremely narrow, less than 

 ipi, and solid distally. An apical archaeopyle appears always to be present. The 

 processes give a reflected tabulation of 6", 6c, 5'", ip, 1"" and a variable number of 

 sulcal processes, commonly 4-6. The detached apical region has not yet been 

 identified. 



This species has been found only in the Lower Cenomanian and is extremely 

 uncommon. 



Remarks. The reticulate nature of the central body and the fibrous processes 

 differentiate H. mantelli sp. nov. from most other species. H. radiculatum sp. nov. is, 

 however, fairly similar but the processes are more deeply divided, there often being 

 2 to 3 sub-parallel branches, and narrower. 



Hystrichosphaeridium latirictum sp. nov. 

 PI. 10, fig. 8 



Derivation of name. Latin, lati, wide and rictum, open mouth — with reference 

 to the considerably expanded distal margins of the processes. 



Diagnosis. Central body ellipsoidal, with smooth or slightly granular wall up to 

 one-quarter \l thick. Processes slender, tubiform, slightly fibrous, expanding 

 considerably distally. Processes open distally and margin circular or quadrate. 



