MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 151 



outer edge of the ledges may be missing " in some specimens ; it is also possible that 

 representatives of two species have been placed together by those authors. For the 

 present, it is considered appropriate that the English forms should be compared with, 

 and not placed within, the species M. ornatum. 



Microdinium setosum sp. nov. 

 PL 16, figs. 9, 10 ; Text-fig. 39 



Derivation of name. Latin, setosus, bristly ; referring to the spines on the 

 sutures. 



Diagnosis. A Microdinium having an ovoidal shell with somewhat flattened 

 antapex. The epitract considerably smaller than the hypotract ; a broad cingulum, 

 weakly spiral, divides them. Tabulation ?i', oa, 6", 6c, 6'", ip, 1"" : two additional 

 plates present in ventral region between two ends of cingulum. Sutures bearing low 

 crests from which arise spines of moderate length, simple and well spaced ; shell 

 surface minutely but densely granular. Apex characteristically lost in the pylome 

 formation. 



Holotype. Geol. Surv. Colin, slide PF.3046(2). Chalk, H.M. Geological 

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

 (basal Cenomanian). 



Dimensions. Holotype : overall length 35^, breadth 30(1 ; shell length 33^, 

 breadth 24^. Range of dimensions : overall lengths 33 to 38jx, breadths 24 to 37^. 



Description. This species is moderately abundant in the lower levels of the 

 Cenomanian of the Fetcham Mill Borehole, over 30 specimens having been seen. The 

 apex is presumed to have consisted of a single plate, but it was lost in the majority 

 of specimens seen. In a few, the archaeopyle " lid " was still attached, but too 

 buckled for its character to be determined. 



Six precingular and six postcingular plates are present, the former being consistent- 

 ly smaller than the latter in the proportion that the epitract is smaller than the 

 hypotract. Plates 1'" and 2'" are reduced : a posterior intercalary plate separates 

 plate 2'" from the antapex. As in M. cf. ornatum, plate ip and the antapical plate 

 are of comparable size and in some specimens look more like paired antapical plates. 



The cingulum is broad, not hollowed, and very weakly spiral, its two ends scarcely 

 differing in antero-posterior position. It is divided into six cingular plates ; two 

 ventral plates lie between its two ends and divide the sulcus into an epitractal and a 

 hypotractal portion. 



The crests are low and bear simple, flexuous spines, the crest margin being scalloped 

 between spine bases. The surface bears a uniform, dense cover of minute granules. 



Remarks. Microdinium setosum sp. nov. differs from M. ornatum in crest 

 character, detail of tabulation, and granular surface. It is placed in Microdinium 

 on the basis of general structure and apical archaeopyle : possession of only one apical 

 plate was presumed but not confirmed. 



