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A fauna consisting of about 200 species of mostly benthonic Foraminifera and 9 of Thecamoebina occurs along the west coast of Trinidad and out into the eastern Gulf of Paria. Three environmental zones roughly paralleling the coastline are recognized: tidal zone, nearshore zone (from the tidal zone out to 2 fathoms), and offshore zone (from 2 to 18 fathoms). Assemblages of the tidal zone consist predominantly of arenaceous species. The arenaceous fraction decreases progressively from the tidal through the nearshore to the offshore zone. It changes character from mostly Haplophragmoides and Trochammina (with minor amounts of fresh-water Thecamoebina) in the tidal zone to predominantly Ammobaculites in the nearshore zone, and predominantly Textularia in the offshore zone.

In assemblages of the nearshore and offshore zones calcareous species predominate. The calcareous fraction decreases progressively from the offshore through the nearshore to the tidal zone.

Although the various species of Streblus (with one exception) and of Elphidium are restricted to one or two zones, these two genera are found abundantly in all three zones. In addition to these genera, the calcareous fraction consists mainly of Rosalina and Palmerinella in the tidal zone. The calcareous fractions of the nearshore and offshore zones together differ from the tidal zone in the additional presence of miliolids, lagenids, nonionids, and buliminids. The nearshore and offshore zones are further distinguished from each other in that the latter includes planktonic species but lacks Rosalina and Palmerineila.

Study of distribution elsewhere of the species characteristic of the three zones shows similarity of the tidal zone fauna to those of shallow brackish environments from various parts of the world. The species characteristic of the nearshore and offshore zones seem to be, with a few exceptions such as certain species of Streblus, an immigrant fauna from the surrounding areas that have normal marine conditions. The immigrant species show evidence of modification in the abnormally small size and extreme fragility of most of the specimens.

Microenvironments of the tidal and nearshore zones in two estuaries are described in detail.

Ten species are described as new, 160 are referred to already known forms, and 40 remain indeterminate. The new species are: Eggerella humboldti, Fissurina agassizi, Haplophragmoides bonplandi, Lagena böglundi, Miliammina pariaensis, Quinqueloculina goësi, Rosalina sagrai, Spiroloculina anderseni, S. guppyi, and Streblus limnetes.

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