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Hemiplacophora
From Williams et al., 2017:
[Hemiplacophora, Cookson and Eisenack, 1965a, p. 125
Type species: Hemiplacophora semilunifera, Cookson and Eisenack, 1965a (pl.14, figs.4–5)]
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Original description: [Cookson and Eisenack, 1965]:
Diagnosis:
Shell broadly oval to almost spherical, dorsoventral, the surface divided into field-like areas by incomplete semicircular ledges. The apex has four small fields, two larger dorsal and two very small ventral ones; there is one large antapical field. The dorsal surface is characterized by four large, approximately semicircular fields, the long axes of which are parallel to the long axis of the shell, two immediately beneath the apex and two adjacent to the antapex; the equatorial are is without ledges. On the ventral surface the ledges are irregularly arranged and vary in number and in outline from short to long and from straight to wavy, curved or even almost circular. The ledges have knob-like or much thickened bases and thin, continuous or perforated sort of wing-like expansions with crenulate margins. The shell opens by an apical archaeopyle.
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Modified description:
Stover and Evitt, 1978, p. 51-52:
Synopsis:
Cysts proximochorate, subspherical; paratabulation gonyaulacacean, indicated by incomplete penitabular ridges and archeopyle; paracingulum lacks ridges; archeopyle apical, Type tA.
Description:
Shape: Subspherical.
Wall relationships: Autophragm only.
Wall features: No parasutural features. Penitabular ridges incomplete; smooth, faintly ornamented, occasionally perforate, of approximately uniform height with smooth to slightly ragged crests; autophragm between ridges faintly and uniform ornamented.
Paratabulation: Indicated by incomplete penitabular ridges and archeopyle; gonyaulacacean, formula: 4`, 5``, Xc, 5```, 1p, 1````. On precingular and postcingular paraplates, the penitabular ridges tend to be missing on areas nearest the paracingulum; five postcingular paraplates interpreted as
corresponding to plates 2``` to 6```.
Archeopyle: Apical, Type tA; principal archeopyle suture zigzag; operculum free; penitabular ridges confined to peripheral area.
Paracingulum: Position indicated by absence of penitabular ridges. Parasulcus: Broad, somewhat triangular area devoid of ridges below parasulcal notch.
Size: Intermediate.
Affinities:
Hemiplacophora differs from Schematophora in having incomplete penitabular ridges. On Schematophora, most of the paraplates have complete penitabular ridges except for those adjacent to or within the parasulcus.
[Hemiplacophora, Cookson and Eisenack, 1965a, p. 125
Type species: Hemiplacophora semilunifera, Cookson and Eisenack, 1965a (pl.14, figs.4–5)]
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Original description: [Cookson and Eisenack, 1965]:
Diagnosis:
Shell broadly oval to almost spherical, dorsoventral, the surface divided into field-like areas by incomplete semicircular ledges. The apex has four small fields, two larger dorsal and two very small ventral ones; there is one large antapical field. The dorsal surface is characterized by four large, approximately semicircular fields, the long axes of which are parallel to the long axis of the shell, two immediately beneath the apex and two adjacent to the antapex; the equatorial are is without ledges. On the ventral surface the ledges are irregularly arranged and vary in number and in outline from short to long and from straight to wavy, curved or even almost circular. The ledges have knob-like or much thickened bases and thin, continuous or perforated sort of wing-like expansions with crenulate margins. The shell opens by an apical archaeopyle.
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Modified description:
Stover and Evitt, 1978, p. 51-52:
Synopsis:
Cysts proximochorate, subspherical; paratabulation gonyaulacacean, indicated by incomplete penitabular ridges and archeopyle; paracingulum lacks ridges; archeopyle apical, Type tA.
Description:
Shape: Subspherical.
Wall relationships: Autophragm only.
Wall features: No parasutural features. Penitabular ridges incomplete; smooth, faintly ornamented, occasionally perforate, of approximately uniform height with smooth to slightly ragged crests; autophragm between ridges faintly and uniform ornamented.
Paratabulation: Indicated by incomplete penitabular ridges and archeopyle; gonyaulacacean, formula: 4`, 5``, Xc, 5```, 1p, 1````. On precingular and postcingular paraplates, the penitabular ridges tend to be missing on areas nearest the paracingulum; five postcingular paraplates interpreted as
corresponding to plates 2``` to 6```.
Archeopyle: Apical, Type tA; principal archeopyle suture zigzag; operculum free; penitabular ridges confined to peripheral area.
Paracingulum: Position indicated by absence of penitabular ridges. Parasulcus: Broad, somewhat triangular area devoid of ridges below parasulcal notch.
Size: Intermediate.
Affinities:
Hemiplacophora differs from Schematophora in having incomplete penitabular ridges. On Schematophora, most of the paraplates have complete penitabular ridges except for those adjacent to or within the parasulcus.