Sedentary—stationary in life, not moving from place to place.
Sediment—material that has been deposited by settling from a transportation agent such as water or air; typically composed of weathered [rock] fragments.
Sedimentary rock—rocks formed from the accumulation and lithification of sediments.
Segment—one of the parts into which a body naturally separates or is divided; for example, segments of arthropods or annelid worms.
Septal—pertaining to the [septum].
Septum (plural, septa)—a dividing wall or partition; in fusulinids, a partition between chambers in the [fusulinid] shell; in corals, one of the radiating, [longitudinal], [calcareous] plates located within the [corallite]; in cephalopods, the [transverse] partitions between the chambers.
Series—the rocks formed during an epoch; the time-stratigraphic term next in rank below a [system].
Serrate—notched like a saw.
Sessile—animal attached to the sea floor more or less permanently.
Silica—an oxide of silicon (SiO₂).