bubble shell

from The Century Dictionary.

• noun A shell of the family Bullidæ and genus Bulla, of an oval form, with the outermost whorl involving all the others. Species are numerous in tropical and warm seas. See cuts under Bulla.

from the GNU version of the Collaborative International Dictionary of English.

• (Zoöl.) A marine univalve shell of the genus Bulla and allied genera, belonging to the Tectibranchiata.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology A marine univalve shell of the genus Bulla and allied genera, belonging to the Tectibranchiata molluscs.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun marine gastropod mollusk having a very small thin shell