ONISCIA. Sow. (G. B.) _Fam._ Purpurifera, Lam. Entomostomata, Bl.--_Descr._ Oblong, sub-ovate, slightly turbinated, cancellated; spire short; aperture elongated; terminating anteriorly in a very short, recurved canal; outer lip thickened, denticulated within; inner lip spread over a portion of the body whorl, granulated.--_Obs._ The granulated inner lip is the principal character by which this genus is distinguished from Cassidaria. In Oniscia the canal is not so produced. O. oniscus, fig. 409. ONUSTUS. Humph. A genus proposed by Humphrey and adopted by Swainson who describes it thus: "Shell trochiform; the surface irregular, and often covered with extraneous bodies, cemented and incorporated with the calcareous substance of the shell; the under part of the body whorl flattened or concave, umbilicate. O. Solaris. Mart. 173. f. 1700, 1701. Indicus. Ib. 172. f. 1697. 1698." It is probable, from the above description, that Mr. Swainson intended to include Trochus agglutinans of authors. (Genus Phorus, Montf.) Fig. 360. OPERCULAR. Of, or belonging to, the operculum. A term applied to the valves which compose the operculum of multivalve shells, as distinguished from the parietal valves, or those which are arranged circularly and form the body of the shell. OPERCULINA. D'Orb. A genus of microscopic Foraminifera. OPERCULUM. (_A cover or lid._) The plate or plates with which many molluscous animals enclose the aperture of their shells, when retired within them. The operculum is sometimes horny, as in Trochus; testaceous or shelly, as in Turbo. It is spiral when from a central or sub-central nucleus, the successive layers take a revolving direction, as in Trochus. It is concentric or annular when the outside edge of each layer entirely surrounds the preceding one. It is unguiculated, when the laminæ are placed side by side, as in Purpura. The opercula of multivalve shells are composed of two or four pieces, which are called the opercular valves. The shelly or membranaceous plate with which some of the animals enclose the aperture of their shells, during the wintry part of the year, for the purpose of protecting them while in a torpid state, and which they get rid of by dissolving the edges, when preparing to emerge from their temporary retirement, must not be considered as the operculum, as it does not belong to or form part of either the animal or its shell, but is produced for the occasion by a secretion of the animal, being deposited in a soft state and subsequently hardening. It is called the epiphragm, and may easily be distinguished from the true operculum by the texture, and by the circumstance of their being soldered to the edge of the aperture. The operculum, on the contrary, is moveable, and is always composed of a