Fig. 1. Alar prolongations and involuteness in lamellar foraminifers. A: planispiral-involute shell with a long apertural face, as in Archaias. Note the apertures in the face of both alar prolongations. B: shape of single (penultimate) spiral chamber with long alar prolongations forming a vortex. C: Random sections of "Peneroplis" glynnjonesi Henson, Lower Oligocene, Iran. Transmitted light. Note foramina in the alar septa. D-E: Transition from planispiral-involute to annular growth. D: external, lateral view of subsequent chambers, showing the ultimate spiral chamber with its alar prolongation and subsequent annular chambers. E: all chambers in equatorial section including the tightly coiled nepiont. F-G: Involuteness and evoluteness in lamellar foraminifers: an accurate definition is whether or not perforate walls (double arrow) cover the next whorl. Axial section, schematic, not to scale. Note the numerous, outer lamellas (green) enveloping the total exposed surface of the previous shell (compare "lamellation"). The distribution of the inner lamella (red) covering the previous whorl hasno significance in the definition of involuteness. H-I: Nummulites incrassatus De La Harpe. Upper Eocene, Northern Italy. H: not quite centered axial section. Red: lumen of alar prolongation, yellow: lumen of equatorial chamber. I: oblique section almost perpendicular to shell axis. Note the obliqueness of the intersections of the alar prolongations. af: apertural face; alp: alar prolongation; anch: annular chamber; ax: shell axis; ch: chamber; f: foramen; il: inner lamella; mc: marginal cord; ol: (numerous) outer lamella; p: pore; per: periphery (of spiral shell); s: septum; spch: spiral chamber; sulc: sulcus; v: vortex; wsut: whorl suture (spiral suture); (Hottinger, 2006; fig. 7  CC/BY-NC-SA)
- according to Hottinger (2006):
MATURO-EVOLUTE – planispiral-involute shells tending to become evolute in mature growth stages.
Remarks: Term introduced by Banner & Hodgkinson (1991), exemplified by Heterostegina depressa. However, in that species and many other nummulitids, the degree of involuteness changes in relation to the depth at which the individual lived. Microspheric specimens tend toward maturo-evoluteness because their ontogeny is much longer than that of the permanently involute megalospheric generation of the same species. See also: and involute in lamellar-perforate foraminifera.
Banner & Hodgkinson (1991), A revision of the foraminiferal subfamily Heterostegininae, Revista Española de Micropaleontologia, Madrid, vol. XXIII, N° 2, p. 101-140
Hottinger (2006), Illustrated glossary of terms used in foraminiferal research. Carnets de Géologie, Memoir 2, ISSN 1634-0744
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