40 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Diagnosis. A species of Hystrichosphaera with well developed crests. Gonal 

 processes reduced to supports for crests. Periphragm of each reflected plate area 

 smooth and extremely thickened. Reflected tabulation typical for genus. 



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

 cal Survey Borehole, Fetcham Mill, Surrey at 670 feet depth. Upper Cretaceous 

 (Cenomanian). 



Dimensions. Holotype : diameter of central body 44 by 46[x, height of crests up 

 to I4fi. Range : diameter of holotype 36-46(1, height of crests up to 14(1. Number 

 of specimens measured, 4. 



Description. Only gonal processes are present and these do not extend beyond 

 the limits of the crests. The processes are usually simple but may terminate in a 

 small bifurcation. The periphragm of the crests is typically slightly granular 

 whereas that of the central body is always smooth. The thickened periphragm on 

 the central body may be up to 3 - 5(x thick. 



This is a rare species, recorded only from the Middle and Upper Cenomanian of 

 England. 



Remarks. High crests enclosing the processes are present in two other species of 

 Hystrichosphaera : H. cingulata (Wetzel) and H. pterota (= Cymatiosphaera pterota 

 Cookson & Eisenack). However H. crassimurata sp. nov. is readily distinguishable 

 from these two species by the thickened areas of the periphragm on the central body. 



Hystrichosphaera crassipellis Deflandre & Cookson 

 PL 1, figs. 7, 8 



1954. Hystrichosphaera crassipellis Deflandre & Cookson, text-fig. 5. 



1955. Hystrichosphaera crassipellis Deflandre & Cookson ; Deflandre & Cookson : 265, pi. 6, 

 figs. 2, 3 ; text-fig. 20. 



1961. Hystrichosphaera crassipellis Deflandre & Cookson ; Gerlach : 177, pi. 27, fig. 5 ; 

 text-figs. 16-18. 



Description. H. crassipellis possesses a thick central body wall (up to 6-5^. in 

 thickness) which is coarsely reticulate. The reticulation is rather irregular having 

 from almost circular to polygonal fields. The processes are gonal, subconical in 

 shape and commonly bifurcate with bifurcating extremities. The paired cingular 

 processes are joined by a membrane. The crests are proximal, reasonably well 

 developed and may be reticulate, especially at their outer edges. H. crassipellis is 

 a rare species occurring throughout the Cenomanian of England. 



Material (Figured). Geol. Surv. Colin., slide PF.3033(2). Lower Chalk, H.M. 

 Geological Survey Borehole, Fetcham Mill, Surrey, at 730 feet depth. Upper 

 Cretaceous (Cenomanian). 



Dimensions. Figured specimen : diameter of central body 45 by 4651, length of 

 processes 12-15(1, overall diameter 64-67(1. Range of Cenomanian specimens : 



