Source : Webster's Revised Unabridged Dictionary (1913)

Tethys \Te"thys\, n. [NL., fr. Gr. ? an oyster, or ? a kind of
   ascidian.] (Zo["o]l.)
   A genus of a large naked mollusks having a very large, broad,
   fringed cephalic disk, and branched dorsal gills. Some of the
   species become a foot long and are brilliantly colored.

Source : WordNet®

Tethys
     n 1: (Greek mythology) a Titaness and sea goddess; wife of
          Oceanus
     2: type genus of the family Aplysiidae [syn: {Aplysia}, {genus
        Aplysia}, {genus Tethus}]