Marginal cord

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Fig. 1. Marginal cord in nummulitids. A-B: Assilina ammonoides (Gronovius), Recent, Gulf of Aqaba, Red Sea. SEM graphs after Hottinger, 1977. A: Detail of marginal cord in a shell broken in the axial plane. Double arrow B - B indicates the approximate position of the cut in the cast of Fig. B. B: Detail of marginal cord in an epoxy resin cast that was cut approximately in the equatorial plane (as indicated in Fig. A), showing canal and chamber lumina after dissolution of shell. White arrows indicate direction of growth. C: Nummulitic limestone from Steinbach, Einsiedeln, Swiss Helvetic Alps, Lower Eocene (Cuisian). Transparent light micrograph of section perpendicular to bedding plane. 1: Assilina sp. of A. ammonea- praespira phyletic line, with particularly large and prominent marginal cord; 2: Nummulites spp.; 3: Assilina sp. of A. spira phyletic line; 4: Assilina sp. of A. exponens phyletic line; 5: Asterigerina cf. rotula (Kaufmann): no marginal cord; ch: chamber lumen; isc: intraseptal canal system; mc: marginal cord; mcc: marginal cord canals; sf: septal flap; st: stolon; sulc: sulcus canal; (Hottinger, 2006; fig. 64 [1] CC/BY-NC-SA)


Definition

  • according to Hottinger (2006):

MARGINAL CORD – a thickened shell margin produced by a peripheral system of numerous longitudinal anastomosing grooves and adjacent imperforate elongated ridges and islands of an inflational ornament type. Grooves are closed by secondary lamination and converted to an anastomosing bundle of peripheral tubular canals. The marginal cord is the diagnostic feature of the family Nummulitidae.





References

Hottinger (1977), Foraminifères operculiniformes, Mémoires du Muséum National d'Histoire Naturelle, Paris, (Série C, Sciences de la terre), t. XL, 159 p. + 66 pls.

Hottinger (2006), Illustrated glossary of terms used in foraminiferal research. Carnets de Géologie, Memoir 2, ISSN 1634-0744

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