From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]