Disparities between ancient and modern occurrences of Cyclammina spp., both in abundance and in the nature of the associated fauna, indicate that the bathymetric distribution of the genus Cyclammina in the Gulf of Mexico has changed with time. An examination of Gulf Coast fossil assemblages reveals that this distribution became more restricted during the late Pliocene, normal occurrences in the younger sediments being limited to upper bathyal and deeper zones. In the older section, the distribution extended into the neritic environment. This change was accompanied by a marked decrease in the relative and absolute abundance of Cyclammina spp. in the younger sediments.

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