Serpula (in Conchology). A worm-like marine shell.
Serrated. Having points like a saw.
Sigillaria. Fossil plants found in the coal-measures.
Silex, Silica. The name given by Mineralogists to a pure earth, more commonly spoken of as flint, and, when crystallized, called rock-crystal.
Silt. The name usually given to the muddy deposit found at the bottom of running streams.
Silurian. The name given by Sir R. Murchison to an important series of fossiliferous rocks well developed in, and first described from, a district in Wales and Shropshire formerly inhabited by the Siluri, a tribe of Ancient Britons.
Siphuncle. A small tube passing through an orifice in the septum of a chambered shell.
Sphenopteris. Fossilfern (leaf wedge-shaped).
Spheroid. Having a shape nearly resembling that of a sphere or globe.
Spirifer. An extinct bivalve.