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Impagidinium modicum
Impagidinium modicum (Brideaux and McIntyre, 1975) Jan du Chene et al., 1986
Originally Leptodinium, subsequently (and now) Impagidinium.
Holotype: Brideaux and McIntyre, 1975, pl.5, figs.10-13; Jan du Chene et al., 1986, pl.59, figs. 4-6
Locus typicus: Horton River Formation, Section CR14A-68, District of Mackenzie, Canada
Stratum typicum: Middle Albian.
Age: Middle Albian
Original diagnosis: Brideaux and mcIntyre 1975, p. 22: Leptodinium modicum
Pentagonal, but rounded, outline; length to width ratio close to l.0. Two smooth wall layers; periphragm thin, forming entire, low sutural crests reflecting a tabulation of 4", 6", 6c, 5"", lp, 1""; reflected plate 1" elongate; 4" elongate and small. Cingulum offset two to three cingular widths; sulcus sinuous and inclined to the right. Archeopyle precingular, formed by loss of reflected plate 3"; operculum detached.
Original description.: Brideaux and mcIntyre 1975, p. 22: Leptodinium modicum
The periblast outline is pentagonal, but rounded; the rounded apex is without an apical horn or prominence. The antapex is slightly flattened. The total length to width ratio of the periblast, excluding crests, is close to l.0. The epitract length is equal to or slightly less than that of the hypotract. Two wall layers are present. The endophragm is smooth and thin, about 1Á thick in optical section. The periphragm is smooth, thin, and closely appressed to the endophragm, except where the periphragm forms low, entire crests along the reflected plate boundaries and at the periphery of the cingular and sulcal regions. The crests are 3 to 6Á high and often so strongly folded as to make determination of reflected tabulation almost impossible. The archeopyle is precingular and is formed by loss of reflected plate 3". The operculum, detached in archeopyle formation, is trapezoidal with somesomewhat rounded corners. The sinistral cingulum varies from 4 to 6Á in width and is outlined by a pair of cingular sutures; six reflected precingular p]ates are present. Where the cingulum is interrupted by the sulcus, the ends of the cingulum are offset from 2 to 3 cingular widths. The sulcus is sinuous, a shallow S in shape, and inclined from left to right. Anteriorly, the sulcus reaches about halfway onto the epitract where it narrows and meets reflected plate l". Posteriorly, the sulcus widens somewhat and extends almost to the antapex where it meets reflected plate l"". The reflected tabulation is difficult to demonstrate on a single specimen because of the strongly folded and contorted sutural crests. From composite observations, the reflected tabulation is determined as: 4", 6", 6c, 5"", lp, 1"". Reflected plate 1" is elongate and 4" is narrow and reduced; of the precingular series, reflected plate 5"" is the smallest. Reflected plate l"" is quadrate to trapezoidal in shape.
Dimensions. Holotype: Length, 45µ; width, 50µ (excluding crests; dorsal surface uppermost, specimen tilted). Range (20 measured specimens) - Periblast length, 50-65µ; periblast width, 53-58µ; endoblast length, 45-58µ; endoblast width, 48-50µ.
Comparison. Leptodinium modicum sp. nov. differs from other species placed in Leptodinium (Davey et al., 1969) in that it possesses low, entire crests, an offset cingulum, inclined sulcus and distinct reflected tabulation pattern.
Occurrence. Horton River Formation (Middle Albian).
Originally Leptodinium, subsequently (and now) Impagidinium.
Holotype: Brideaux and McIntyre, 1975, pl.5, figs.10-13; Jan du Chene et al., 1986, pl.59, figs. 4-6
Locus typicus: Horton River Formation, Section CR14A-68, District of Mackenzie, Canada
Stratum typicum: Middle Albian.
Age: Middle Albian
Original diagnosis: Brideaux and mcIntyre 1975, p. 22: Leptodinium modicum
Pentagonal, but rounded, outline; length to width ratio close to l.0. Two smooth wall layers; periphragm thin, forming entire, low sutural crests reflecting a tabulation of 4", 6", 6c, 5"", lp, 1""; reflected plate 1" elongate; 4" elongate and small. Cingulum offset two to three cingular widths; sulcus sinuous and inclined to the right. Archeopyle precingular, formed by loss of reflected plate 3"; operculum detached.
Original description.: Brideaux and mcIntyre 1975, p. 22: Leptodinium modicum
The periblast outline is pentagonal, but rounded; the rounded apex is without an apical horn or prominence. The antapex is slightly flattened. The total length to width ratio of the periblast, excluding crests, is close to l.0. The epitract length is equal to or slightly less than that of the hypotract. Two wall layers are present. The endophragm is smooth and thin, about 1Á thick in optical section. The periphragm is smooth, thin, and closely appressed to the endophragm, except where the periphragm forms low, entire crests along the reflected plate boundaries and at the periphery of the cingular and sulcal regions. The crests are 3 to 6Á high and often so strongly folded as to make determination of reflected tabulation almost impossible. The archeopyle is precingular and is formed by loss of reflected plate 3". The operculum, detached in archeopyle formation, is trapezoidal with somesomewhat rounded corners. The sinistral cingulum varies from 4 to 6Á in width and is outlined by a pair of cingular sutures; six reflected precingular p]ates are present. Where the cingulum is interrupted by the sulcus, the ends of the cingulum are offset from 2 to 3 cingular widths. The sulcus is sinuous, a shallow S in shape, and inclined from left to right. Anteriorly, the sulcus reaches about halfway onto the epitract where it narrows and meets reflected plate l". Posteriorly, the sulcus widens somewhat and extends almost to the antapex where it meets reflected plate l"". The reflected tabulation is difficult to demonstrate on a single specimen because of the strongly folded and contorted sutural crests. From composite observations, the reflected tabulation is determined as: 4", 6", 6c, 5"", lp, 1"". Reflected plate 1" is elongate and 4" is narrow and reduced; of the precingular series, reflected plate 5"" is the smallest. Reflected plate l"" is quadrate to trapezoidal in shape.
Dimensions. Holotype: Length, 45µ; width, 50µ (excluding crests; dorsal surface uppermost, specimen tilted). Range (20 measured specimens) - Periblast length, 50-65µ; periblast width, 53-58µ; endoblast length, 45-58µ; endoblast width, 48-50µ.
Comparison. Leptodinium modicum sp. nov. differs from other species placed in Leptodinium (Davey et al., 1969) in that it possesses low, entire crests, an offset cingulum, inclined sulcus and distinct reflected tabulation pattern.
Occurrence. Horton River Formation (Middle Albian).