Both fossil and living foraminifera have been investigated in this study. Late Cretaceous age foraminiferal assemblages have been used to examine the relationship between high-frequency sea level change, biotic response and paleoceanography, in the U.S. Western Interior Sea. Living planktic foraminifera were also studied in order to test a novel hypothesis on the evolution of cellular organelles known as fibrillar bodies. The major findings of this dissertation are: (1) The third-order Greenhorn tectono-eustatic cycle (late Cenomanian-middle Turonian) in the Cretaceous Western Interior Basin is superposed by fourth-order relative sea level cycles and fifth-order parasequences that are reflected in foraminferal assemblages and carbonate content of shales and mudrocks. The study interval includes the Cenomanian/Turonian (C/T) boundary event (93.3 Ma) and the highest stand of sea level in the western interior during early Turonian time. Calcareous benthic foraminiferal assemblages show marked shifts in taxon dominance which is interpreted as ecologic replacement indicating change from a food-controlled to an oxygen-controlled environment. Acmes of the genus Gavelinella are related to the initiation of fourth-order transgressive episodes early in the third-order transgression and late highstand phases of the Greenhorn Cycle. The rapid proliferation of this taxon is thought to be a response to pulses of food. Warm, oxygen-poor Tethyan waters spread across the WIS during the late transgression and highstand phases of the Greenhorn Cycle. A rapid ecologic shift to Neobulimina dominance is the benthic foraminiferal response to this Tethyan incursion. The relative abundance of Neobulimina shows a highly significant correlation to carbonate content (p $<$ 0.01), which is a proxy for warm, more normal marine water masses entering the WIS from the south. Tethyan water masses were replaced by those of Boreal affinity and agglutinated benthic foraminiferal assemblages during the regressive phase of the Greenhorn Cycle. Variations in benthic foraminiferal assemblage composition mirror changes in water mass salinity, oxygenation, circulation and productivity. Foraminiferal data from the western margin of the WIS support GCM models of estuarine circulation. (2) Fibrillar bodies are organelles present in the cytoplasm of all planktic foraminifera. I propose that they may have originated as symbiotic bacteria. Those in Pulleniatina obliquiloculata are squat to elongate ovoids and elongate rods with a length of 2-15 $\mu$m and a diameter of 2-5 $\mu$m. One, sometimes two such bodies (possibly products of binary fission) are bounded by a membrane-like vacuolar structure. They have an electron-lucent (DNA-containing?) central region, ribosome-like and storage-like granules, and are commonly enclosed within a vacuole. (Abstract shortened by UMI.)