Electron microscopy of benthic calcareous Foraminifera reveals the presence of a calcite layer on both external and internal test surfaces. This layer is analogous to the calcite crust which has been reported on external surfaces of planktonic Foraminifera. The crust was observed on two granular-walled species, Nonionella scapha and Nonionella miocenica, and on two radial-walled species, Cancris indicus and Bolivina spissa. On these four species, the crust is a very thin layer which occurs only on specimens from the greater depth ranges. Where no crust is present, test surfaces of granular- and radial-walled species have different appearances at high magnifications. However, crusts may cause surfaces on both groups to appear similar. On the radial-walled species Bolivina rgentea, crust is noticeable on specimens from shallow water and becomes progressively thicker, reaching a maximum thickness of more than 4 µ at the lower end of the depth range. Specimens of B. argentea from a low oxygen environment have thinner crusts than specimens from a normal environment of the same depth. The crust is not a postmortem feature because it is found on specimens which have absorbed a rose bengal stain. The crust is generally thicker on early than on late chambers, indicating its probable secondary origin.

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