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Isthmocystis distincta
Isthmocystis distincta Duxbury, 1979, p.201, pl.1, figs.1-6; pl.2, fig.8; text-figs.3A-B,4.
Holotype: Duxbury, 1979a, pl.1, figs.1-2; text-fig.4; Fensome et al., 1993a, figs.1-2,7 - p.1133.
Locus typicus: Speeton Clay, Speeton, England
Stratum typicum: Valanginian
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G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999 - LPP VIEWER CD-ROM 99.5.
Isthmocystis distincta Duxbury, 1979a. Epicyst hypocyst ratio about 1:2. The parasutural crests on the hypocyst are distally entire, commonly with large perforations in the proximal part. Size: maximum width 85-115 µm, holotype 107 x 80 µm.
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Original description: Duxbury 1979a, p. 201
Diagnosis: Dinoflagellate cysts having a subpentagonal lateral outline and bearing a well-developed apical protrusion. The body surface is smooth to finely granular, and the epitractal to hypotractal length ratio is approximately 1:2. The epitract is devoid of any sutural ornament. Cracks, presumably along plate boundaries, may be present in the epitract when the archeopyle is only partially formed. The epitractal cingular crest is absent. The hypotract displays a reflected tabulation pattern of 6''', 1p, ?1pv, 1'''', outlined by distinctive sutural crests. These crests are distally entire and commonly exhibit large perforations which transect the proximal edges of the crests but not the distal edges.The hypotractal cingular crest is well-developed. The archeopyle is formed by partial or, ultimately, complete detachment of the precingular plate series.
Observed dimensions: Maximum width range 85-115 Ám; specimens measured: 8.
Remarks: The detachment of the precingular plate series in archeopyle formation may be only partial. Usually, sufficient precingular plates are lost to allow 'hingeing' of the remaining part of the epitract at the anterior part of the sulcus. This causes the collapse of the epitract into the hypotractal body cavity (much as in the genus Wanaea Cookson and Eisenack, 1958) and consequent shortening of the cyst. This shortening causes the width to be by far the maximum dimension and the most common orientation to be polar. In polar orientation, I.distincta is most reminiscent of some species of Wanaea. The varying degrees of epitractal shortening in archeopyle formation exhibited by this species render any reference to cyst length in the 'Observed dimensions' section above irrelevant. Consequentely, only the maximum width is noted.
Holotype: Duxbury, 1979a, pl.1, figs.1-2; text-fig.4; Fensome et al., 1993a, figs.1-2,7 - p.1133.
Locus typicus: Speeton Clay, Speeton, England
Stratum typicum: Valanginian
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G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999 - LPP VIEWER CD-ROM 99.5.
Isthmocystis distincta Duxbury, 1979a. Epicyst hypocyst ratio about 1:2. The parasutural crests on the hypocyst are distally entire, commonly with large perforations in the proximal part. Size: maximum width 85-115 µm, holotype 107 x 80 µm.
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Original description: Duxbury 1979a, p. 201
Diagnosis: Dinoflagellate cysts having a subpentagonal lateral outline and bearing a well-developed apical protrusion. The body surface is smooth to finely granular, and the epitractal to hypotractal length ratio is approximately 1:2. The epitract is devoid of any sutural ornament. Cracks, presumably along plate boundaries, may be present in the epitract when the archeopyle is only partially formed. The epitractal cingular crest is absent. The hypotract displays a reflected tabulation pattern of 6''', 1p, ?1pv, 1'''', outlined by distinctive sutural crests. These crests are distally entire and commonly exhibit large perforations which transect the proximal edges of the crests but not the distal edges.The hypotractal cingular crest is well-developed. The archeopyle is formed by partial or, ultimately, complete detachment of the precingular plate series.
Observed dimensions: Maximum width range 85-115 Ám; specimens measured: 8.
Remarks: The detachment of the precingular plate series in archeopyle formation may be only partial. Usually, sufficient precingular plates are lost to allow 'hingeing' of the remaining part of the epitract at the anterior part of the sulcus. This causes the collapse of the epitract into the hypotractal body cavity (much as in the genus Wanaea Cookson and Eisenack, 1958) and consequent shortening of the cyst. This shortening causes the width to be by far the maximum dimension and the most common orientation to be polar. In polar orientation, I.distincta is most reminiscent of some species of Wanaea. The varying degrees of epitractal shortening in archeopyle formation exhibited by this species render any reference to cyst length in the 'Observed dimensions' section above irrelevant. Consequentely, only the maximum width is noted.