The Collaborative International Dictionary of English v.0.48:

Tethys \Te"thys\, n. [NL., fr. Gr. ? an oyster, or ? a kind of
   ascidian.] (Zool.)
   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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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