Robulus macomberi, n. sp., has been used as the marker for a biostratigraphic zone in the post-Anahuac Miocene of S. Louisiana under the designation Robulus "43," Robulus "L" or Cristellaria "angular". The only known species which remains in constant association with this marker is Discorbis crosbiei, n. sp. The zone marked by these forms lies stratigraphically between the Cibicides opima Zone above and the Operculinoides sp. Zone below. It can be traced across Louisiana from the vicinity of Lake Pontchartrain westward to Cameron Parish. Offshore it has been noted in wells from the northern Eugene Island area southwestward to Brownsville, Texas, and may possibly occur in the offshore areas of northeastern Mexico.

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