Ultrastructure of the test wall in modern porcelaneous foraminifera: Implications for the classification of the miliolida

New insight into wall ultrastructure has been gained from examination of 130 species of porcelaneous foraminifera. New terms are introduced to describe the porcelaneous wall, including: "porcelain" for the main body of the wall that is comprised of a matrix of randomly oriented needle-shaped crystals; "intrados" for the inner mineralised surface; and "extrados" for the outer mineralised surface. The main test wall may also contain aligned needle-shaped crystals, which form an "infraskeletal framework" where parallel to the test wall, and "infraskeletal windows" where perpendicular to the wall surface. The intrados is constructed of needle-shaped crystals oriented randomly on a plane parallel to the inner surface of the chamber. Its structure is relatively consistent across all studied species and does not change during ontogeny. The porcelain is the fundamental part of the wall that determines appearance (e.g., carinate, costate, striate and pitted ornament). It is primarily constructed of a matrix of randomly oriented needle-shaped crystals, which in some species have embedded sediment particles, and in Pseudohauerina is perforated by a canaliculate network. Within the porcelain in soritid and peneroplid taxa, an infraskeletal framework occurs in chamber and chamberlet walls that forms the basis of the chamber around which the randomly-oriented crystals of the porcelain are placed. In Alveolinella and Borelis the porcelain of lateral chamber walls contains crystals aligned perpendicular to the chambers that form infraskeletal windows, which likely assist in light transmission into the test. The extrados in the studied taxa is constructed of needle-, rod-or plate-shaped crystals that may be aligned or non-aligned. At the genus level there is typically consistency in the structure of the extrados, however in Quinquelocutina and Triloculina variation between species is observed. Examination of wall ultrastructure is a useful character to help understand attempts to split these genera, and more broadly to better understand the phylogenetic links between taxa.