Secreted tests form by organic precipitation by the foraminifera over a basal organic membrane. The are numerous types of secreted tests, the three most common of which are microgranular calcareous, porcelaneous, and hyaline calcareous.
tests are restricted to the late paleozoic. They consist of uncemented,
tightly packed, equidimensional and subspherical grains of calcite.
walls are translucent to opaque magnesian calcite. Thin inner and outer
laminae inclose a relatively thicker interior layer. The outermost veneer
can be tangential crystal laths or even radially arranged laths providing
a needley surface texture. Such a radial arrangement can trap superficial
agglutinated particles, thereby resulting in a foraminifera resembling
a true agglutinating foraminifera.
Hyaline calcareous tests are generally multilamellar and commonly porous. The c-axis of the calcite framework can show a preferred growth orientation in given foraminifera groups.
There are of course exceptional cases of secreted tests that have been documented and may be worth noting. Spirillinina patellina was recognized to have its entire test grown from one optically single calcite crystal. Aragonite tests are another anomaly as far as secreted tests go. The genus Carterina is thought to be composed of spicules of either secreted or agglutinated calcite.