TETRACORALLA
Te`tra*co*ral"la, n. pl. Etym: [NL. See Tetra-, and Corallum.]
(Paleon.)
Defn: Same as Rugosa.
TETRACTINELLID
Te*trac`ti*nel"lid, n. (Zoöl.)
Defn: Any species of sponge of the division Tetractinellida. Also used adjectively.
TETRACTINELLIDA
Te*trac`ti*nel"li*da, n. pl. Etym: [NL., from Gr. (Zoöl.)
Defn: A division of Spongiæ in which the spicules are siliceous and have four branches diverging at right angles. Called also Tetractinellinæ.
TETRAD
Tet"rad, n. Etym: [L. tetras, -adis, Gr. tétrade.]
1. The number four; a collection of four things; a quaternion.
2. (Chem.)
Defn: A tetravalent or quadrivalent atom or radical; as, carbon is a tetrad.