ABSTRACT

The foraminifer Orbitolina occurs profusely on the outcrop in strata of Glen Rose age. The fossils vary in size and shape, probably depending on whether they are microspheric or megalospheric in their initial stage. Their internal structure as well as external features are being studied. For this purpose, many specimens have been etched with dilute hydrochloric and acetic acids, and the results have been quite satisfactory except perhaps for the extreme outer marginal zone of the test. Heretofore, only the specimens with a megalospheric initial stage have been studied in some detail.

Orbitolina, besides its embryonic apparatus, has a very complex and delicate internal structure. This structure is divided into 3 zones: the central complex, reticulate or central zone; the radial zone; and the marginal zone. Preliminary observations with the scanning electron microscope were concentrated on the radial zone, and especially on the marginal zone, since these are always present and play an important role in the morphology of the test.

The radial and marginal zones consist of chambers and chamber passages with connecting tubes, called stolons, which cover and encircle the entire test in successive offsetting layers. This pattern is striking and beautifully delicate. The chamber walls, as well as the chamber passages and their stolons, apparently have been strengthened by the cementation of calcite crystals, as evidenced by the holes left after etching.

After the crystals between the chamberlets and cellules were dissolved by acid, voids or empty spaces remained. These, when observed with transmitted light, appeared to be the features previously described as "partitions" and/or "plates" in axial and horizontal thin sections of Orbitolina.

Without the aid of the scanning microscope, some minute details within Orbitolina would not have been known, or would have been difficult to interpret.