Scanning electron microscopy of tests of Triticites ventricosus and Schwagerina sp. shows that the microgranular wall was secreted rather than agglutinated. Grains range in size from 0.5 to 6.0 mu m, but grains larger than 4 mu m are rare. Evidence that grains were secreted includes tight packing, uniform size and shape, and apparently homogeneous composition. Furthermore, scanning electron microscopy confirms the perforate nature of the antetheca and the nature of the keriothecal wall. Alveoli taper toward the tectum and pass through the tectum as tiny pores, enhancing communication between chambers and with the external environment. The traditional model for the addition of chambers is rejected. Three ways in which new chambers may have been added to the test that are consistent with observations made here: 1) The antetheca may have thickened differentially, creating a substrate to which the keriotheca of the new chamber was attached: 2) Part of the tectum, septum, and keriotheca of the previous chamber may have been resorbed before calcite forming the new chamber was secreted, perhaps necessary because the tectum was an unsuitable substrate on which to attach the keriotheca of the new chamber; 3) A combination of the above may have occurred wherein the antetheca thickened differentially and was resorbed locally, providing a suitable substrate on which to attach the keriotheca of the new chamber. The last model is favored here because it best explains the shape of the septa and the configuration of the keriotheca of the chambers of most specimens. Nevertheless, the other two models are consistent with the morphology of some specimens.