SERTULARIDA (Lat. sertum, a wreath).  An order of Hydrozoa.

SESSILE (Lat. sedo, I sit).  Not supported upon a stalk or peduncle; attached by a base.

SETHAE (Lat. bristles).  Bristles or long stiff hairs.

SIGILLARIOIDS (Lat. sigilla, little images).  A group of extinct plants of which Sigillaria is the type, so called from the seal-like markings on the bark.

SILICEOUS (Lat. silex, flint).  Composed of flint.

SINISTRAL (Lat. sinistra, the left hand).  Left-handed; applied to the direction of the spiral in certain shells, which are said to be “reversed.”

SIPHON (Gr. a tube).  Applied to the respiratory tubes in the Mollusca; also to other tubes of different functions.

SIPHONIA (Gr. siphon, a tube).  A genus of fossil Sponges.

SIPHONOSTOMATA (Gr. siphon; and stoma, mouth).  The division of Gasteropodous Molluscs in which the aperture of the shell is not “entire,” but possesses a notch or tube for the emission of the respiratory siphon.

SIPHUNCLE (Lat. siphunculus, a little tube).  The tube which connects together the various chambers of the shell of certain Cephalopoda (e.g., the Pearly Nautilus).

SIRENIA (Gr. seiren. a mermaid).  The order of Mammalia comprising the Dugongs and Manatees.

SIVATHERIUM (Siva, a Hindoo deity; Gr. therion, beast).  An extinct genus of Hoofed Quadrupeds.

SOLIDUNGULA (Lat. solidus, solid; ungula, a hoof).  The group of Hoofed Quadrupeds comprising the Horse, Ass, and Zebra, in which each foot has only a single solid hoof.  Often called Solipedia.

SPHENOPTERIS (Gr. sphen, a wedge; pteris, a fern).  An extinct genus of ferns.

SPICULA (Lat. spicidum, a point).  Pointed needle-shaped bodies.

SPIRIFERA (Lat. spira, a spire or coil; fero, I carry).  An extinct genus of Brachiopods, with large spiral supports for the “arms.”

SPIRORBIS (Lat. spira, a spire; orbis, a circle).  A genus of tube-inhabiting Annelides, in which the shelly tube is coiled into a spiral disc.

SPONGIDA (Gr. spoggos, a sponge).  The division of Protozoa commonly known as sponges.

STALACTITES (Gr. stalasso, I drop).  Icicle-like encrustations and deposits of lime, which hang from the roof of caverns in limestone.

STALAGMITE (Gr. stalagma, a drop).  Encrustations of lime formed on the floor of caverns which are hollowed out of limestone.

STIGMARIA (Gr. stigma, a mark made with a pointed instrument).  A genus founded on the roots of various species of Sigillaria.

STRATUM (Lat. stratus, spread out; or stratum, a thing spread out).  A layer of rock.

STROMATOPORA (Gr. stroma, a thing spread out; paras, a passage or pore).  A Palaeozoic genus of Protozoa.