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.
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