The pteropod assemblages in 120 post-Wisconsin cores were studied from an area between the Atlantic continental shelf of North America and the Mid-Atlantic Ridge. Pteropod shells are one of the major constituents in the calcareous pelagic sediments at depths ranging from 350 to 4,200 m., particularly along the Mid-Atlantic Ridge, the Bermuda Pedestal, the Blake Plateau, and the Gulf of Mexico.

The subarctic species Limacina retroversa and the temperate species Clio pyramidata are present in the Mid-Atlantic Ridge sediments north of about 45° N. Lat. They indicate the faunal mixing zone between subarctic and warmer North Atlantic Drift waters.

Subtropical species Limacina inflata, L. bulimoides, and Styliola subula are the dominant pteropod species in the middle-latitude sediments of the Mid-Atlantic Ridge and the Bermuda Pedestal, and the overlying water of the Sargasso Sea. The subarctic species was present in the pelagic sediments of the Bermuda Pedestal, but is at present not living in the overlying water; it is inferred from this that subarctic waters invaded the Bermuda region some time ago. The maximum concentration of pteropod shells was found in the pelagic sediments of the Bermuda Pedestal at a depth of approximately 2,200 m. The number of specimens decreases considerably in sediments containing less than 80 per cent calcium carbonate at water depths greater than 3,600 m. No pteropods were found below 4,200 m.

Tropical Creseis acicula is the predominant species on the ocean floor of the Gulf of Mexico and Blake Plateau, and the overlying water of the Gulf Stream currents. The delicate hexagonal surface reticulation of Peraclis reticulata shells were obscured after deposition on the ocean floor; probably this resulted from greater solution of its projecting ridges. Two pteropod species, Peraclis

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triacantha and P. bispinosa, with thin, fragile shells were absent from the sediments but are known to inhabit the deep waters in low and middle latitudes.

The present study shows that pteropods as a group are useful for paleoecological interpretations of their fossil assemblages on the ocean floor.

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