For a number of years a foraminiferal species, commonly designated Robulus "43" by economic paleontologists, has been used to mark a biostratigraphic zone in the Miocene post-Anahuac sedimentary sequence of south Louisiana. In the petroleum industry this species has also been known as Robulus "L," Robulus "4," and Cristellaria "angular." A paper describing and naming this species and its associate Discorbis "4" has been submitted to the Journal of Paleontology for publication (Butler, in press).

Regionally, the Robulus "43" zone lies stratigraphically below the Cibicides carstensi opima and Amphistegina "B" zones and above the Operculinoides sp. zone. The Amphestegina "B" fauna generally occurs 100 to 200 feet above the Robulus "43" zone, but tends to climb stratigraphically in the section along strike and to the southwest. Since Robulus "43" shows less stratigraphic variation than the Amphistegina "B" fauna, it is considered a more reliable regional marker on which to base correlations.

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