The Collaborative International Dictionary of English v.0.48:

Ianthina \I*an"thi*na\, n.; pl. L. Ianthin[ae], E.
   Ianthinas. [NL., fr. L. ianthinus violet-blue, Gr. ?; ?
   violet + ? flower.] (Zool.)
   Any gastropod of the genus Ianthina, of which various
   species are found living in mid ocean; -- called also purple
   shell, and violet snail. [Written also janthina.]
   [1913 Webster]

   Note: It floats at the surface by means of a raft, which it
         constructs by forming and uniting together air bubbles
         of hardened mucus. The Tyrian purple of the ancients
         was obtained in part from mollusks of this genus.
         [1913 Webster]