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Nyktericysta arachnion
Nyktericysta arachnion Bint, 1986
Holotype: Bint, 1986, pl.4, figs.13-15, pl.8, figs.7-8
Locus typicus: Western Interior, U.S.A.
Stratum typicum: Late Albian
Original description: Bint, 1986, p.153
Intermediate to large, dorso-ventrally compressed cysts with a tapering, narrowly blunted apical horn, two tapering, pointed antapical horns, and two broad, blunt lateral horns from which arise short, rounded pre- and postcingular extensions. Left antapical horn is always slightly larger than right. Left lateral horn may be slightly larger in overall size than right; lateral horns are variable in size and may be reduced to low, rounded bulges. On any one lateral horn the precingular extension is often shorter than the postcingular extension.
Cysts two-layered, cornucavate. Endocyst produced into five prominent horns: apical and two antapical horns are long and tapering; lateral horns are broad and blunt and usually do not extend into the precingular extensions but often protrude into the postcingular extensions. In all horn terminations the endocyst is rounded. Pericyst usually encloses small pericoels under all horn tips, but in some specimens the antapical horn tips bear a solid periphragmous projection. In other specimens the antapical pericoels may be enlarged beyond the horn tips. Occasionally the pre- and postcingular projections on the lateral horns may be connected producing a single lateral cornucavation.
Over the cyst surface, the periphragm lies close to the endophragm, separated from it in an indistinct and incomplete reticulate pattern, but otherwise appressed to the endophragm. The separation of the two walls is represented by the muri of the reticulum, and the lumina (about 2-4 µm across) correspond to the sites of appression of the two walls. Endophragm smooth, periphragm finely and densely perforate, with perforations about 0.2 µm across but ranging from 0.1-0.4 µm. Both walls are very thin, about 0.2 µm each.
Paratabulation not expressed. Cingulum indicated by indentation of the lateral horns and faint to distinct transverse lines; sulcus not indicated. Archeopyle apical, operculum adnate ventrally but often broken away. Archeopyle suture weakly angular, incomplete, leaving a mid-ventral attachment area between the operculum and the cyst body.
Size. Length of complete specimen 85(98)112 µm, 8 measurements; length with operculum broken away 67(74)82 µm, 7 measurements; width 63(75)84 µm; 15 specimens measured. Approximate lengths of horns: apical 24-37 µm, precingular 1-7 µm, postcingular 2-8 µm, left antapical 14-35 µm, right antapical 9-23 µm, length of apical pericoel 6-5 µm.
Affinities:
Bint, 1986, p.154: Nyktericysta arachnion is similar to N. davisii which differs in having the periphragm completely appressed to the endophragm and in having solid rather than cavate lateral and antapical horn tips. N. pentagona (Singh 1983) is very similar to N. arachnion (especially to the specimen figured here as Plate 5, fig. 16) but differs in having the periphragm separated from the endophragm over most of the cyst surface, with the two walls in contact only in the cingular region and at the horn tips. Also the reticulation is fainter, less complete, and has larger lumina in N. pentagona. Nyktericysta sp. A is distinguished from N. arachnion in lacking the reticulate separation of the wall layers, in having larger, often connected cornucavations, and in having reduced precingular extensions on the lateral horns.
Holotype: Bint, 1986, pl.4, figs.13-15, pl.8, figs.7-8
Locus typicus: Western Interior, U.S.A.
Stratum typicum: Late Albian
Original description: Bint, 1986, p.153
Intermediate to large, dorso-ventrally compressed cysts with a tapering, narrowly blunted apical horn, two tapering, pointed antapical horns, and two broad, blunt lateral horns from which arise short, rounded pre- and postcingular extensions. Left antapical horn is always slightly larger than right. Left lateral horn may be slightly larger in overall size than right; lateral horns are variable in size and may be reduced to low, rounded bulges. On any one lateral horn the precingular extension is often shorter than the postcingular extension.
Cysts two-layered, cornucavate. Endocyst produced into five prominent horns: apical and two antapical horns are long and tapering; lateral horns are broad and blunt and usually do not extend into the precingular extensions but often protrude into the postcingular extensions. In all horn terminations the endocyst is rounded. Pericyst usually encloses small pericoels under all horn tips, but in some specimens the antapical horn tips bear a solid periphragmous projection. In other specimens the antapical pericoels may be enlarged beyond the horn tips. Occasionally the pre- and postcingular projections on the lateral horns may be connected producing a single lateral cornucavation.
Over the cyst surface, the periphragm lies close to the endophragm, separated from it in an indistinct and incomplete reticulate pattern, but otherwise appressed to the endophragm. The separation of the two walls is represented by the muri of the reticulum, and the lumina (about 2-4 µm across) correspond to the sites of appression of the two walls. Endophragm smooth, periphragm finely and densely perforate, with perforations about 0.2 µm across but ranging from 0.1-0.4 µm. Both walls are very thin, about 0.2 µm each.
Paratabulation not expressed. Cingulum indicated by indentation of the lateral horns and faint to distinct transverse lines; sulcus not indicated. Archeopyle apical, operculum adnate ventrally but often broken away. Archeopyle suture weakly angular, incomplete, leaving a mid-ventral attachment area between the operculum and the cyst body.
Size. Length of complete specimen 85(98)112 µm, 8 measurements; length with operculum broken away 67(74)82 µm, 7 measurements; width 63(75)84 µm; 15 specimens measured. Approximate lengths of horns: apical 24-37 µm, precingular 1-7 µm, postcingular 2-8 µm, left antapical 14-35 µm, right antapical 9-23 µm, length of apical pericoel 6-5 µm.
Affinities:
Bint, 1986, p.154: Nyktericysta arachnion is similar to N. davisii which differs in having the periphragm completely appressed to the endophragm and in having solid rather than cavate lateral and antapical horn tips. N. pentagona (Singh 1983) is very similar to N. arachnion (especially to the specimen figured here as Plate 5, fig. 16) but differs in having the periphragm separated from the endophragm over most of the cyst surface, with the two walls in contact only in the cingular region and at the horn tips. Also the reticulation is fainter, less complete, and has larger lumina in N. pentagona. Nyktericysta sp. A is distinguished from N. arachnion in lacking the reticulate separation of the wall layers, in having larger, often connected cornucavations, and in having reduced precingular extensions on the lateral horns.