Surface Ultrastructure of Some Tests of Recent Foraminiferida from the Dry Tortugas, Florida

SYNOPSIS. The surface ultrastructure of 10 species of benthic Foraminiferida from the Dry Tortugas, Florida, has been investigated using an electron microscope. Tests of miliolid Foraminiferida appear to be made up of at least 2 distinct layers. An outermost pavement-like layer, consisting of a single layer of calcite rhombohedrons, and an underlying "randomly oriented layer" comprise most of the wall thickness. It was possible to differentiate each miliolid species on the basis of a statistical analysis of calcite rhombs making up the pavement-like layer. Pseudopores were found on the tests of Miliolinella subrotunda (Montagu) and Archaias angulatus (Fichtel and Moll). None of these pseudopores completely penetrated the wall, and all floored in the "randomly oriented layer." It is believed that these structures are formed at time of calcification, rather than secondarily at some later time.
Calcification in Foraminiferida appears to be a biphasic process similar to that found in most other organisms. Groups of crystals were discernible on surfaces of most porcelaneous and "radiate" calcareous species. This suggests that parallel growth of crystals may be an important process in calcification of many Foraminiferida.
A sieve-plate was found in a canal paralleling the test's surface in Cymbaloporetta squammosa (d'Orbigny).