. Bulletin of the British Museum (Natural History) Geology Supplement. MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 117 intercalary plate is present. The single antapical plate is large and polygonal ; the crests surrounding it are supported at their junctions by spines. A posterior ventral plate may be present, but if so, its anterior boundary is ill-defined. The cingulum is relatively narrow and pronouncedly laevo-rotatory, its two ends differing in antero-posterior position by three times its width. The sulcus is markedly sigmoidal ; a median dorso-ventral section would thus cut both ends of
. Bulletin of the British Museum (Natural History) Geology Supplement. MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 117 intercalary plate is present. The single antapical plate is large and polygonal ; the crests surrounding it are supported at their junctions by spines. A posterior ventral plate may be present, but if so, its anterior boundary is ill-defined. The cingulum is relatively narrow and pronouncedly laevo-rotatory, its two ends differing in antero-posterior position by three times its width. The sulcus is markedly sigmoidal ; a median dorso-ventral section would thus cut both ends of the cingulum. Six cingular plates appear to be present ; the posterior end of the cingulum is separated from the sulcus by a small crescentic plate, poorly marked or indistinguish- able in many specimens and thus excluded from the diagnosis. The crests crossing furrows lack denticles. An irregular scatter of tubercles is present on the surfaces of the plates ; the number, density and situation of these tubercles varies greatly between individuals. A precingular archaeopyle is usually present, formed by loss of plate 3". Remarks. The diagnosis of Gonyaulacysta helicoidea is emended to include reference to the tabulation. This species is numerous in the assemblages from 39 and 42-5 metres depth in the Speeton Clay ; although the English specimens are markedly smaller, there can be no doubt that they are conspecific with the Australian species. A closely comparable species, Gonyaulacysta cretacea (Neale & Sarjeant 1962) is present in somewhat earlier horizons (99-25 metres—Hauterivian) in the West Heslerton boring. This differs from G. helicoidea only in having a more markedly polygonal outline and in lacking tubercles. It seems probable that G. cretacea is ancestral to G. helicoidea. The form from the Lower Cretaceous of New South Wales, figured by Deflandre & Cookson (1956, pi. 1, fig. 6) as IGonyaulax sp. indet., may well be attributable to this Please
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