1. Foraminifera et cetera have been studied from approximately 400 surface-sediment samples in Mississippi Sound, the marsh and estuaries on the adjacent mainland, the barrier sand islands bounding Mississippi Sound on the south, and the open gulf outside the barrier islands.

2. The microfaunas in this area can be divided into four geographic and ecologic facies, as follows.
a. The open-gulf facies occurs outside the barrier islands and is characterized by being composed mostly of calcareous Foraminifera, principally Nonionella atlantica, Cibicidina strattoni, Elphidium, various Miliolidae, Discorbis, and "Rotalia" beccarii var. Ostracoda are restricted to this facies.
b. The sound facies contains arenaceous Foraminifera, principally Ammobaculites.
c. The estuary facies is characterized by an abundance of Ammobaculites exilis and Ammosclaria fluvialis. Miliammina fusca is abundant.

d. The marsh facies is characterized by Ammoastuta inepta, Arenoparrella mexicana, Leptodermella, Trochammina comprimata, T. macrescens, Urnulina, Haplophragmoides subinvolutum, and Recurvoides sp. Miliammina fusca is abundant.

3. A few subdivisions of these facies appear to be somewhat distinctive, as follows.
a. Nearshore barrier-island faunas, generally characterized by low populations of Elphidium, Miliolidae, or "Rotalia."
b. Barrier-island lake faunas may contain a mixture of marsh and sound species, in some places with open-gulf species.
c. Inner marsh samples are differentiated by abundance of Leptodermella and Urnulna.

4. There is mixing of faunas, as follows.
a. The open-gulf faunas occur inside inlets between barrier islands and at some places behind the islands, where there is mixing with the sound fauna.
b. The most abundant marsh and estuary species, Miliammina fusca, is mixed with the sound fauna near marsh areas, principally near the mainland.
c. Marsh species occur in the estuary faunas, presumably due to mixing, and are most abundant where the estuaries are narrow.

5. The contrast between the open-gulf and the sound faunas is believed to be caused by the islands acting as a barrier to the invasion of the open-gulf water mass into the sound. This is aided by high runoff into Mississippi Sound.

The presence of open-gulf faunas in the sound suggests that the routes of invasion of open-gulf water are along the bottom. Distributions of sound faunas in the open gulf probably reflect average hydrographic conditions.

6. In the San Antonio Bay area, near Corpus Christi, Texas, there are open-gulf and bay faunas similar to those in the Mississippi Sound area. Elphidium spp. and "Rotalia" beccarii vars. occur in the San Antonio Bay fauna. This is believed to be related to the low runoff so that most of the bay is undiluted open-gulf water. Few marsh and/or estuary species occur in the bay fauna.

Atchafalaya Bay, a high runoff area with no barrier island, contains a mixture of bay, open-gulf, and marsh and/or estuary species.