Holocene bottom samples from the northwest Gulf of Mexico, examined for their Ostracoda faunas, yielded a total of 171 species. A large majority of them have never been described. The number of species per sample varied from 2 to 40; the richest and most diversified faunas occur in middle to outer shelf environments. Numerous species exhibit rather restricted depth ranges, and distinctly different faunal assemblages characterize the various bathymetric environments from shallow freshwater lakes and bayous down to the abyssal depths of the Sigsbee Deep. A bathymetric chart illustrates that at the generic level there exists a marked diversity of faunal composition at different depths.

A pronounced relation has been noted between the size of 2 common species of Echinocythereis and the depth at which they were collected. A similar connection was observed in 12 species of the genus Krithe.

The results of this study will be of considerable use in evaluating the paleoecologic significance of the rich, ostracod assemblages commonly found in the clastic Tertiary and Upper Cretaceous deposits of the Gulf Coast area.

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