5. Rims

For heterococcoliths a very useful division can be made between the rim and central-area . The rim is predominantly formed of cyclic structures with high rotational symmetry.

5.1 Parts of rims

Each of these parts may be formed of a single cycle of elements, part of a cycle or several cycles.
Shield broad (sub-)horizontal structure (placoliths).
Tube (sub-vertical structure between two shields (placoliths).
Wall (sub-)vertical structure not associated with shields (muroliths).
Flange (sub-)horizontal protrusion from rim.
Collar (sub-)vertical protrusion from rim (may occur on proximal or distal surface).
Crown discontinuous/beaded collar.

parts of rim

5.2 Directions on the rim

Largely based on Black (1972)
Radial direction in the surface of the baseplate perpendicular to its margin: Inward-outward - toward-away from centre.
Tangential direction in the surface of the baseplate parallel to its margin: Clockwise/dextral/right, anticlockwise/sinistral/left senses of direction as seen in distal view. We recommend: use of clockwise/anticlockwise as the clearest of these terms for general purposes. Use of dextral/sinistral when it is wished to particularly emphasise that this is the orientation as seen in distal view.
Vertical direction perpendicular to the baseplate: Up/down distal-proximal directions.
Flare and taper divergence of orientation from horizontal/vertical in the radial direction. Flare surfaces diverge upward, producing obconical/funnel-shaped bodies. Taper surfaces converge upward, producing conical bodies.

rim directions

5.3 Element arrangement as seen in side view

Imbrication/inclination divergence from horizontal in the tangential direction. Imbrication is applicable to a cycle of elements, inclination to individual elements.
offset of upper part of element from lower.
Imbrication angle angle of contact-surface from the horizontal. High-angle - sub-vertical contact-surfaces. Low-angle - sub-horizontal contact-surfaces.
Zeugoid rim rim with high-angle imbrication, and without distinct shields. (Alternative terms loxolith rim, zygodiscid rim, see appendix).

element arrangement

5.4 Element arrangement as seen in plan view

Obliquity horizontal divergence from radial direction. (Alternative term precession, see appendix).
Dextral/sinistral obliquity deflection from radial of outer part of element relative to inner part, as seen in distal view. Note that elements will show opposite apparent senses of obliquity in distal and proximal view. This can be described as follows: a dextrally oblique cycle displays clockwise obliquity in distal view but anti-clockwise obliquity in proximal view.
Butting elements with simple (sub-)radial sutures.
Interlocking elements with complex sutures.
Overlapping elements with low angle oblique sutures (N.B. This pattern has occasionally been described as imbrication, but we prefer to use imbrication for description of vertical structures).

5.5 Identification of elements

For description and discussion, the various elements/cycles of elements need to be identified. This is best done by reference to the location of the elements using the set of orientation and structure terms given above. Examples are given in the figure below. Element shape is not recommended as an alternative since it is easily altered - by diagenesis, intra-specific variation and evolution.
examples of heterococcolith rim structure and terminology
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