Ianthina
Ianthina , noun
[New Latin, from Latin ianthinus violet-blue, Greek {not transcribed}; {not transcribed} violet + {not transcribed} flower.]
(Zoology) Any gastropod of the genus Ianthina, of which various species are found living in mid ocean; -- called also purple shell, and violet snail.
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.