from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A lid or flap covering an aperture, such as the gill cover in some fishes or the horny shell cover in snails or other mollusks.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A covering flap or lidlike structure in plants and animals, such as a gill cover
from the GNU version of the Collaborative International Dictionary of English
- n. The lid of a pitcherform leaf.
- n. The lid of the urnlike capsule of mosses.
- n. Any lidlike or operculiform process or part.
- n. The fold of integument, usually supported by bony plates, which protects the gills of most fishes and some amphibians; the gill cover; the gill lid.
- n. The principal opercular bone in the upper and posterior part of the gill cover.
- n. The lid closing the aperture of various species of shells, as the common whelk. See Illust. of Gastropoda.
- n. Any lid-shaped structure closing the aperture of a tube or shell.
from The Century Dictionary and Cyclopedia
- n. A lid or cover; in natural history, a part, organ, or structure which forms a lid, flap, or cover.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a hard flap serving as a cover for (a) the gill slits in fishes or (b) the opening of the shell in certain gastropods when the body is retracted
The snail in the middle with the aperture of its shell sealed with an operculum is Pomatias elegans.
In the case of P. elegans, the foot has to come out first to move the operculum, which is attached to the foot, out of the way.
These latter filaments do not appear externally, and indeed a membrane, termed the operculum (Fig. 2, op), is developed from the front of each series of branchial apertures, Fig. 3.
Well, I think it's a Batillaria minima; I found it among Batillaria minima and the microsculpture of its shell, its operculum and the morphology of its head look like those of Batillaria minima.
Car passes the living room, an operculum widens into sluice — red filigree arches and gray fish mouth cleaves the heel of air — and seals again within its glistened sleeve.
Ammonites had an operculum (pl. opercula) as well.
You can see the snail's operculum deep inside the aperture.
In the 1st photo the aperture is mostly covered by the thin operculum, while in the 2nd photo, the snail's foot is pushing the operculum arrow out of the way.
So, if the snail that I identified as Assiminea succinea didn't have an operculum, it would likely to be a stylommatophoran, although, as Tim noted, it did not have 2 pairs of tentacles.
But why does H. concavum, which doesn't have an operculum, have to put its foot out first?