6. Central-areas

Central-area structures are very variable in structure and form, and individual species often have unique structures. As a result terminology needs to be rather generalised to avoid obscure terms for unique structures. We have tried here to define a set of simple terms for describing the most common features.

6.1 Structural types

Conjunct formed from crystal-units of the rim structure. E.g. Gephyrocapsa (bridge and grill), Helicosphaera sellii (bar), Kamptnerius (plate), Watznaueria biporta (bar). (Alternative term optically continuous structure, see appendix). {Young 1992a}
Disjunct formed from crystal-units discrete from the rim structure. E.g. Arkhangelskiella (plate), Coccolithus pelagicus (bar), Helicosphaera seminulum (bar), Watznaueria britannica (bar). (Alternative term optically discontinuous structure, see appendix). {Young 1992a}

conjunct vs. disjunct structures

6.1a Bar structures in cross-polars (added 2019)

The structure/appearance in xpl of bars and crosses is a significant feature, especially for Mesozoic taxa such as Zeugrhabdotus. To aid consistency of terminology the following terms are suggested.
Simplebar, or similar, appearing to be formed of a single unit
Split perpendicular to lengthbar, or similar, appearing to be formed of two units, meeting in the middle
Divided longitudinallybar, or similar, appearing to be formed of two parallel sub-bars separated by an extinction line
Complexbar, or similar, with more complicated structure, appearing to be formed of numerous sub-units

6.2 Orientation in profile

Basal occurring on the proximal surface.
Elevated occurring above the proximal surface.
Vaulted cone-shaped, rising from the rim toward the centre.
Planar flat, not vaulted.


6.3 Orientation in plan view

Transverse parallel with short axis of (elliptical) coccolith.
Longitudinal parallel with long axis of (elliptical) coccolith.
Diagonal inclined relative to axes. We reccomend that the angle should be measured from the transverse direction, hence:
Low angle near to transverse direction;
High angle near to longitudinal direction.
NB Some authors use the opposite convention, i.e. measure angle from longitudinal direction, so care needs to be taken when using the literature.
Dextral/sinistral inclined to the right/left of the long-axis as seen in distal view. N.B. As with element obliquity the terms dextral/sinistral are preferred for describing orientations which appear different in proximal and distal view.
Relative width width of central-area relative to rim width:
Wide central-area width >2x rim width;
Normal central-area width 1-2x rim width;
Narrow central-area width <1x rim width.

bar orientations

6.4 Structures spanning central-area

Arm part of crossbar, bridge or cross running from centre of coccolith to edge of central-area. (alternative terms limb, spoke, see appendix).
Bar any elongate central-area structure. N.B. This is a general term. When it is useful to be more specific terms such as longitudinal bar, cross-bar, and arm can be used. (Alternative term jugum, see appendix).
Blanket covering of small elements on distal side of central-area (e.g. Helicosphaera, Coccolithus).
Bridge elevated bar spanning the central-area (e.g. Gephyrocapsa).
Cross-bar bar spanning the central-area.
Cross pair of cross-bars meeting in centre.
Axial cross (abb. +), cross-bars longitudinal and transverse.
Diagonal cross (abb. X) cross-bars diagonal - may be symmetrical or asymmetrical relative to the axes.
Offset cross cross with an offset between the arms of one, or both, of the crossbars (e.g. Chiasmolithus).
Foot broadening of bar as it meets the rim (e.g. Cruciplacolithus tenuis).
Lateral bar bar running from rim to a cross bar (e.g. Retecapsa).

cross types

6.5 Structures closing central-area

Central opening opening at centre of coccolith, may be spanned by bars or other central-area structures, but not by a continuous structure such as a grill or plate.
Closed central-area central-area without a central opening.
Grill system of bars closing central-area (e.g. Emiliania).
Net mesh-like structure closing central-area (e.g. Reticulofenestra, Cribrosphaerella). (Alternative term cribrate central-area, see appendix).
Open central-area central-area without any structures.
Plate continuous or nearly continuous structure closing central-area.
Perforated plate plate with perforations (e.g. Arkhangelskiella).

closing structures

6.6 Processes

Calyx flaring structure at tip of process (e.g. Podorhabdus, Papposphaera).
Boss low process, height similar to or less than width (alternative term knob, see appendix).
Process general term for any structure rising from the central-area.
Protrusion broad low process, with height similar to width, and width near that of entire central-area. Types:
Conical cone-shaped protrusion (e.g. Acanthoica);
Sacculiform sac-like protrusion with more or less rounded upper part (e.g. Algirosphaera). (N.B. labiatiform has been used for the elongate double-lipped sacculiform protrusions, see appendix).
Spine elongated process, height greater than width. (Alternative term column, see appendix). Types:
Styliform {Halldal and Markali 1955} - spine tapers toward the distal end;
Claviform {Halldal and Markali 1955} - spine has blunt end, without calyx. (N.B. helatoform has been used for nail-shaped processes, see appendix);
Calicate - spine is surmounted by a calyx.
Salpingiform {Braarud et al. 1955a, 1955b} - spine (or protrusion) trumpet-shaped (e.g. Discosphaera).
Stem part of process below calyx.
Cavity wide opening within process (e.g. Podorhabdus grassei, Algirosphaera robusta).
Canal narrow opening running along length of process.
Proximal pore opening of canal, on proximal side of central-area.

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