Radial symmetry—see [Symmetry].
Reef—a mound-like or ridge-like elevation of the sea bottom which almost reaches the surface of the water, composed primarily of organic material and commonly formed by reef-building animals, such as corals and oysters.
Replacement—type of fossilization whereby hard parts of organisms are removed by solution accompanied by almost simultaneous deposition of other substances in the resulting voids; mineralization.
Respiration—the process of oxygenation.
Rock—an aggregation of one or more minerals.
Rock-unit—divisions of rocks based on definite physical and lithologic characteristics and not defined on the basis of geologic time alone; groups, formations, members.
Rudistid—a [Cretaceous] [pelecypod] that does not exhibit the typical clam or oyster shape; many are cone-shaped, resembling corals.
Saddle—in cephalopods, the forward flexure (curved toward the [aperture]) of the [suture] or [septum].
Scaphopod—an exclusively marine mollusk with a single-valved tusk-shaped shell; member of class Scaphopoda, [phylum] Mollusca.
Scavenger—an animal that feeds on organic refuse.