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Palaeostomocystis triquetra
Palaeostomocystis triquarta Brideaux, 1977
Now Fromea. Originally Palaeostomocystis, subsequently (and now) Fromea.
Holotype: Brideaux, 1977, pl.8, figs.1-2
Locus typicus: Richardson Mountains, District of Mackenzie, Canada
Stratum typicum: Barremian
Original diagnosis: Brideaux, 1977, p,20-21
Shape: Autocyst length equal to or greater than maximum width, pear-shaped overall; epicyst more or less straight-sided in its upper part but widening abruptly in its lower part just before the pericingulum; hypocyst bilobed and forming marked antapical prominences; hypocyst straight to slightly invaginated between the
prominences; the lobations commonly compressed and folded; dorso-ventral compression.
Phragma: Autophragm about 1.0-1.5 Ám thick; sculpture of variably shaped grana, short rows of fused grana, vermlculate ridges, and rarely, cones or spines; sculpture often longitudinally orientated, except transversely orientated in what is interpreted as the position of the perisulcus; sculpture in the perisulcal region consisting of fine, anastomosing striae; grana less than 1.0 Ám wide and high, vermiculate ridges less than 1.0 Ám high and about 0.5 Ám wide, striae less than 0.5 Ám high and wide.
Paratabulation: Evidence for determinable paratabulation lacking; subangular outline of archeoyle pronounced on some specimens indicating the presence of apical paratabulation.
Archeopyle/operculum: Archeopyle apical, formed by the loss of a subcircular to subangular part of the apex, the archeopyle opening indistinctly to markedly angular. Operculum flat, rounded to subrounded and slightly angular in outline, generally detached, but occasionally in place or lying in the autocoel. Formula A.
Pericingulum/perisulcus: Position of the pericingulum marked indistinctly and discontinuously by parallel rows of larger and more widely spaced grana, 1.0-1.5 Ám wide and high; the pericingulum lying just below the point at which the autocyst widens abruptly. Position of perisulcus interpreted as lying between the two antapical prominences, and indicated by the trapezoidal-shaped region of autophragm bearing transversely orientated striae.
Variation/other features: Autophragm sculpture varies conslderably, many specimens possessing only discrete grana without fusion or orientation; grana commonly rounded but in place trianguloid to rectanguloid in outline. Occasional specimens possess longitudinally orientated striae, the striae swinging around to a transverse orientation in the assumed perisulcal region.
Affinities:
Brideaux, 1977, p.21: Palaeostomocystis triquetra is distinguished from Palaeostomocystis expolita by its sculpture and from other published species of the genus by its trilobate shape and autophragm sculpture. Horologinella sp. indet. of
Cookson and Eisenack, 1962, is smaller, although superficially similar in shape and sculpture, but details of the apical region are difficult to determine on the published
figure. Trigonopyidia ginella (Cookson and Eisenack) Downie and Sarjeant, 1965 possesses a triangular shape and apical opening, but differs in having an internal body, more pronounced development of the antapical projections, a wider apical
opening in both wall layers, and is three dimensionally a tetrahedron with a triangular base (E.J. Kidson, pers. com., 1976).
Now Fromea. Originally Palaeostomocystis, subsequently (and now) Fromea.
Holotype: Brideaux, 1977, pl.8, figs.1-2
Locus typicus: Richardson Mountains, District of Mackenzie, Canada
Stratum typicum: Barremian
Original diagnosis: Brideaux, 1977, p,20-21
Shape: Autocyst length equal to or greater than maximum width, pear-shaped overall; epicyst more or less straight-sided in its upper part but widening abruptly in its lower part just before the pericingulum; hypocyst bilobed and forming marked antapical prominences; hypocyst straight to slightly invaginated between the
prominences; the lobations commonly compressed and folded; dorso-ventral compression.
Phragma: Autophragm about 1.0-1.5 Ám thick; sculpture of variably shaped grana, short rows of fused grana, vermlculate ridges, and rarely, cones or spines; sculpture often longitudinally orientated, except transversely orientated in what is interpreted as the position of the perisulcus; sculpture in the perisulcal region consisting of fine, anastomosing striae; grana less than 1.0 Ám wide and high, vermiculate ridges less than 1.0 Ám high and about 0.5 Ám wide, striae less than 0.5 Ám high and wide.
Paratabulation: Evidence for determinable paratabulation lacking; subangular outline of archeoyle pronounced on some specimens indicating the presence of apical paratabulation.
Archeopyle/operculum: Archeopyle apical, formed by the loss of a subcircular to subangular part of the apex, the archeopyle opening indistinctly to markedly angular. Operculum flat, rounded to subrounded and slightly angular in outline, generally detached, but occasionally in place or lying in the autocoel. Formula A.
Pericingulum/perisulcus: Position of the pericingulum marked indistinctly and discontinuously by parallel rows of larger and more widely spaced grana, 1.0-1.5 Ám wide and high; the pericingulum lying just below the point at which the autocyst widens abruptly. Position of perisulcus interpreted as lying between the two antapical prominences, and indicated by the trapezoidal-shaped region of autophragm bearing transversely orientated striae.
Variation/other features: Autophragm sculpture varies conslderably, many specimens possessing only discrete grana without fusion or orientation; grana commonly rounded but in place trianguloid to rectanguloid in outline. Occasional specimens possess longitudinally orientated striae, the striae swinging around to a transverse orientation in the assumed perisulcal region.
Affinities:
Brideaux, 1977, p.21: Palaeostomocystis triquetra is distinguished from Palaeostomocystis expolita by its sculpture and from other published species of the genus by its trilobate shape and autophragm sculpture. Horologinella sp. indet. of
Cookson and Eisenack, 1962, is smaller, although superficially similar in shape and sculpture, but details of the apical region are difficult to determine on the published
figure. Trigonopyidia ginella (Cookson and Eisenack) Downie and Sarjeant, 1965 possesses a triangular shape and apical opening, but differs in having an internal body, more pronounced development of the antapical projections, a wider apical
opening in both wall layers, and is three dimensionally a tetrahedron with a triangular base (E.J. Kidson, pers. com., 1976).