. Bulletin of the British Museum (Natural History) Geology Supplement. MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 149 smooth, denticulate, or bearing spines of varying lengths ; alternatively sutures marked by lines of closely set spines. Shell surface smooth, granular, punctate or tuberculate. Archaeopyle formation by loss of single apical plate. Type species. Microdinium ornatum Cookson & Eisenack 1960a. ?U. Aptian- Turonian : Australia. Remarks. At the time when it was proposed, this genus was technically invalid, since its tabulation falls within the limits specified for Gonyaulax. Ho
. Bulletin of the British Museum (Natural History) Geology Supplement. MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 149 smooth, denticulate, or bearing spines of varying lengths ; alternatively sutures marked by lines of closely set spines. Shell surface smooth, granular, punctate or tuberculate. Archaeopyle formation by loss of single apical plate. Type species. Microdinium ornatum Cookson & Eisenack 1960a. ?U. Aptian- Turonian : Australia. Remarks. At the time when it was proposed, this genus was technically invalid, since its tabulation falls within the limits specified for Gonyaulax. However, Microdinium is now recognized to be a cyst genus and, regarded as such, becomes a valid entity, since its tabulation does not accord with that of Gonyaulacysta either in the original definition of Deflandre or as here emended. Representatives of this genus are relatively frequent in the English Cenomanian. The generic diagnosis is emended to accommodate variations in morphology exhibited by these forms. Microdinium differs from the majority of fossil dinoflagellate cysts in having an apical archaeopyle ; from Meiourogonyaulax and Xiphophoridium in having only a single apical plate ; and from Glyphanodinium in having six precingular plates. Microdinium cf. ornatum Cookson & Eisenack PL 16, figs. 3-6 ; Text-fig. 38 1960a. Microdinium ornatum Cookson & Eisenack : 6, pi. 2, figs. 3-8, text-figs. 2-4. Material (Figured). Geol. Surv. Colin, slide (i). Chalk, Geologi- cal Survey Borehole, Fetcham Mill, Surrey, at 670 feet depth. Upper Cretaceous (Upper Cenomanian). Dimensions. Figured specimen : length 40^, breadth 32^. Range length , breadth ^.. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London : BM(NH)
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