Beam

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Fig. 1. The structure of Orbitopsella: a simple exoskeleton and a pillared endoskeleton in a discoidal shell: Orbitopsella dubari Hottinger, Bou Dahar, Eastern Morocco, Middle Lias. Transmitted light micrographs.A: oblique section of complete microspheric specimen; B: oblique section of microspheric specimen. The septa of the thickened margin are cut tangentially and reveal the alternating pattern in the disposition of foramina on the septal face. C: oblique tangential section of a microspheric specimen at a low angle to the equatorial plane showing a part of the disc with its exoskeleton (restricted to beams). Note the large open spaces, the lateral annular passages (arrows), that separate exoskeleton and endoskeleton. D: Oblique centered section of megalospheric specimen. Note the structured wall of the embryo, that shows it to be a sphaeroconch. E: Transverse section (parallel to the axis of coiling) of a megalospheric specimen. The septum in this tangential section reveals the alternating pattern of the apertures. F: schematic model of structure after Hottinger, 1967; not to scale; green: exoskeleton; brown: endoskeleton; a: aperture; ap: annular passage; b: beam; f: foramen; p: pillar; pr: sphaeroconch; s: septum; (Hottinger, 2006; fig. 72 [1] CC/BY-NC-SA)
Fig. 2. Alveolar exoskeleton and polygonal network. A-C: simple alveolar layer in Everticyclammina virguliana (Koechlin), Mechra Klila, Northeastern Morocco, Uppermost Jurassic. A: stereograph, schematic, not to scale. B: tangential section. Note the large size of the alveoles in a postseptal position. C: para-equatorial, non-centered section showing septa and the basal coat at the bottom of the chamber, resembling a basal layer. E-G: polygonal network in Spirocyclinidae. E: Choffatella tingitana Hottinger, megalospheric generation, in tangential section near to the equatorial plane. Note the clear differentiation of beams and rafters. D: stereograph of spirocyclinid polygonal network. Note the curved pigeon holes in preseptal position which in axial section might be mistaken for foramina. F-G: extension of beams into a corrugated sheet that replaces endoskeletal pillars in Hottingertidae. F: stereograph representing a part of an axial section. Not to scale. G: Alveosepta powersi (Redmond), Northeastern Morocco, Upper Jurassic. Equatorial section of megalospheric specimen. alv: alveoles; b: beam; bl: basal layer; corr: corrugated median extension of beams; f: foramen; ph: pigeon holes; s: septum; sf: supplementary foramina; sph: sphaeroconch; ssut: septal suture. Arrows: direction of growth. After Hottinger, 1967.; (Hottinger, 2006; fig. 45 [2] CC/BY-NC-SA)
Fig. 3. Henson's "subepidermal partitions" (1948, text-figs. 6-7). A: axial sections of uniserial cones (as in Orbitolinidae) with (a) "primary subepidermal plates" and (b) with "primary and secondary subepidermal plates". B: basal sections of radial zone showing in addition to "radial plates" (a) "thick, straight radial partitions, (b) "thin radial partitions in zigzag" and (c) "subepidermal plates thickening inward". Current interpretation given in red: b: beam (perpendicular to septum); e: epiderm; f: foramina (in Orbitolinines forming a crosswise-oblique pattern); r: rafter (parallel to septum); s: septum; sl: septulum (may fuse with beam); (Hottinger, 2006; fig. 19 [3] CC/BY-NC-SA)

Definition

  • according to Hottinger (2006):

BEAM - an exoskeletal main partition of the chamber lumen, perpendicular to the chamber septum and to the lateral chamber wall. In discoidal shells often separated from an endoskeleton by an empty space in the chamber (annular passage). May fuse with endoskeletal elements such as septula, particularly in verbeekinids, orbitolinids and cuneolinids. May occur as unique exoskeletal element (Orbitopsella, Fig. 1.) or in combination with minor, shorter exoskeletal elements producing a subepidermal polygonal network (Spirocyclinidae, Fig. 2.).


Remarks: Davies (1930) distinguished "primary" or "major" (1939) partitions in the description of orbitolinids in opposition to "secondary" or "minor" elements. Henson (1948) separated "subepidermal plates" from main partitions (the latter being of endoskeletal nature and therefore to be called septula nowadays; compare Henson's fig. 7, reproduced here as Fig. 3. Henson's (1948) subepidermal plates, synonymous with Silvestri's "trabecole perpendicolari" (1932), were subdivided again into "transverse" and "parallel" partitions corresponding to Davies' major and minor elements. Henson's general term "subepidermal partition" may include main partitions (= septula), transverse partitions (= beams) and parallel partitions (= rafters), where septula and beams fuse to produce (mostly radial) chamber compartments. In order to distinguish partitions according to their origin, Hottinger (1967) introduced particular terms for partitions exclusively of exoskeletal origin, i.e. "poutre" for major and "poutrelle" for minor partitions, translated into English in 1978 as "beam" and "rafter" (Balken and Bälkchen in German). For illustration see exoskeleton.





See also:

References

Davies (1930), The genus Dictyoconus and its allies: a review of the group, together with a description of three new species from the lower Eocene beds of northern Baluchistan, Transactions of the Royal Society of Edinburgh, vol. 56, p. 485-505.

Davies (1939), An early Dictyoconus and the genus Orbitolina: their contemporaneity, structural distinction and respective natural allies, Transactions of the Royal Society of Edinburgh, vol. 59, p. 773-790.

Henson(1948), Larger imperforate Foraminifera of south-western Asia, Families Lituolidae, Orbitolinidae and Meandrosipinidae, British Museum (Natural History), London, 127 p. + 16 pls.

Hottinger (1967), Foraminifères imperforés du Mésozoïque marocain. Notes et Mémoires du Service géologique, Rabat, N° 209, p. 5-168

Hottinger (1978), Comparative anatomy of elementary shell structures in selected larger Foraminifera, In: Hedley R.H. & Adams C.G. (eds.), Foraminifera. Volume 3.- Academic Press, London, p. 203-266.

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

Silvestri (1932), Foraminiferi del Cretaceo della Somalia, Paleontographica Italica, Pisa, vol. XXXII (1931, Nuova Serie, vol. II), p. 143-204.


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