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Cyclammina cancellata Brady shows marked variations of diameter and thickness in Holocene sediments from depths of 500 m to more than 3,500 m in the Peru-Chile Trench area. The mean diameter increases consistently to a maximum of 5 mm at 2,000 m. Fluctuations between 4 and 5 mm follow from 2,000 to 3,500 m, and a decreasing trend characterizes deeper samples. The mean thickness increases steadily downward to about 3,500 m where a slight decrease sets in.

Using mean diameter, thickness, and a ratio between both, populations of this study can be characterized as to depth zones. Thus, small and relatively thick forms appear at about 500 m; larger and proportionally thinner forms live deeper than 1,000 m; large but relatively thick specimens characterize depths of about 2,000 to 3,500 m; and somewhat smaller and thicker ones are typical for depths below 3,500 m.

Temperature may be the principal factor affecting size, because it increases markedly to about 2,000 m--which coincides with the greatest size change in the populations. In deeper water other factors may play a role. Oxygen, salinity, and nitrate values do not show significant trends. Pressure alone is not directly involved, because off southern California similar size variations occur in different depth zones.

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