Considerable new information has been obtained on the bathymetry and fauna of several calcareous banks on the continental shelf of the northwest Gulf of Mexico. A compilation of depths to tops and bases of 130 banks on the shelf revealed primary modes of depth occurrences at 9, 31, and 45 fathoms. These modes correspond with well developed terraces at the same depths on the banks studied in detail. Several of the banks were characterized by semi-ring depressions, steep central pinnacles, and flat terraces at several depths.

The fauna collected on these banks at depths of between 24 and 31 fathoms are characteristic of intertidal depths in the West Indies and Caribbean. Many of the mollusks from these banks exhibited sub-specific differences from the species which have been collected from their normal range in the West Indies. Despite the existence of dead reef coral, no living forms were collected. Living lithothamnioids were common on all banks. Evidence suggests that the tops of these banks were in shallow intertidal waters at a time when over-all water temperatures were warmer than at present, and when populations of these faunas were continuous to the West Indies. As sea-level and hydrographic conditions changed, the original fauna became isolated and the corals which appear to form the primary organ c capping ceased their growth, leaving horizontal surfaces. The presence of large crystals of gypsum and inclusions of anhydrite from the flanks of one bank may have a bearing on the original topographic high at this place.