Previous classifications and generic determinations of planktonic Foraminifera have been based variously on features of gross test morphology, including chamber shape and arrangement; test shape and ornamentation; and apertural number, form, and position. Certain of these features appear to be convergent adaptations for a planktonic existence, and hence are unreliable for determining natural relations.

Skeletal, chemical, and mineralogical composition, microcrystalline structure, and septal lamellar characters appear not to be environmentally affected, and hence probably best reflect relationships. Planktonic Foraminifera, superfamily Globigerinacea, have perforate tests constructed of radially built calcite crystals, with c-axes perpendicular to the test surface. Although the surface texture may be modified by secondarily deposited material (crusts, pustules, and rugosities), the primary texture is determined by three different types of radial microstructure. These characteristic wall features of the enrolled planktonic Foraminifera provide a useful basis for family delineation.

A wall constructed of closely packed identical crystals, resulting in a predominantly smooth surface, is found in the Cretaceous families Planomalinidae, Schackoinidae, Rotaliporidae, and Globotruncanidae, and also is characteristic of the Cenozoic Globorotaliidae and Hantkeninidae. The Cenozoic family Catapsydracidae differs in having thicker rod-like crystals surrounded by finer ones between which occur the test perforations. In the Globigerinidae, thicker crystals are greatly elongated as spines extending far beyond the general test surface, and very thin crystals surround the crystal spine bases. This characteristic wall structure, distinguishable even where elongate spines are broken, is the latest to appear in the geologic record.

The Globorotaliidae and Catapsydracidae appear to have descended directly from Cretaceous stocks and the Hantkeninidae from the Globorotaliidae in the Eocene. Cenozoic Globigerinidae are not closely related to the morphologically similar Cretaceous hedbergellids, but were derived from the Catapsydracidae during Eocene time.

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