ultrastructure -- The detailed structure of a specimen, such as a cell, tissue, or organ, that can be observed only by electron microscopy. Also called fine structure. In eggshell, ultrastructure refers to the three-dimensional arrangement of mineral crystals and organic matter. It is described in terms of calcite or aragonite mineralogy and the transition between different zones of organization within the shell. Distinct zones of organization are called ultrastructure zones.
unconformity -- Any interruption of the continuity of a depositional sequence.
uniramious -- Among arthropods, uniramous refers to appendages that have only one branch. Insects, centipedes and millipedes, and their relatives are uniramous arthropods; land-living chelicerates such as scorpions, spiders,and mites are also uniramous but probably descended from ancestors with biramous appendages. Contrast with biramous.
upwelling -- The raising of benthic nutrients to the surface waters. This occurs in regions where the flow of water brings currents of differing temperatures together, and increases productivity of the ecosystem.