Fossil individuals of Bolivina seminuda, studied with an SEM, reveal a tripartite test-wall ultrastructure composed of (1) an underlying organic lining with pore structures, (2) a radial calcitic layer bounded by organic membranes, and (3) a calcitic surface veneer. A calcification model for Bolivina seminuda is proposed where the inner organic lining is precipitated first. Overlying this is a layer composed of radial calcite with the crystallographic C-axes aligned perpendicular to the test wall. This calcite is laid down in successive packets bounded above and below by organic membranes. Above this is a surface veneer composed of randomly oriented calcite rhombs. This model differs from some proposed models by the absence of a randomly oriented calcite rhomb layer incor orated with each packet of radial calcite and by attributing the surface veneer to inorganic precipitation.

The surface veneer is interpreted as diagenetic in origin and not precipitated by the organism. The diagenetic calcification patterns at first enhance preexisting surface sculpture and then gradually mask it. This coating makes the different phenotypes of Bolivina seminuda similar to one another and to other species in appearance. The effects of this calcification must be considered in taxonomy, biostratigraphy, and paleoenvironmental interpretations.

SEM studies of the ultrastructure of Bolivina seminuda indicate which test-wall parameters are controlled by genetics, environment, and postdepositional (or postmortem) history. The raw data may be of use in subsequent investigations of paleoenvironments, taphonomy, and postdepositional (or postmortem) history.

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