Part 53

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 series of successive layers, corresponding with the growth of the shell.

OPIS. Defr. A genus described by De Blainville as consisting of species of Trigonia which have the umbones sub-spiral, with a large, striated tooth on the hinge. Opis cardissoides, Trigonia, Lam. Opis similis, Sow. Min. Con. pl. 232. f. 2.

ORAL. (Os, oris, mouth.) Applied to that part of a shell which corresponds with the mouth of the animal, but very seldom used in this sense.

ORBICULA. Lam. (Orbis, an orb.) Fam. Brachiopoda, Lam. Palliobranchiata, Bl.--Descr. Inequivalve, irregular, sub-orbicular, compressed, attached by a fibrous substance passing through a fissure near the centre of the lower valve; upper valve patelliform, with the umbo central; muscular impressions four in each valve, semilunar. South America and West Indies.--Obs. Discina, Lam. is an Orbicula. Crania is known from this genus by having no fissure in the lower valve, but being attached by its substance. Hipponyx has only two muscular impressions in each valve. O. lævis, fig. 201.

ORBICULAR. (Orbiculus, a little orb.) Of a round or circular form.

ORBICULINA. Lam. A genus of microscopic Foraminifera.

ORBIS. Lea. A minute fossil, described as "orbicular, with flat quadrate whorls and aperture square," in other respects resembling Solarium. O. Rotella, fig. 355, 356.

ORBITINA. Risso. A genus said to be established upon the nuclei of two land shells.

ORBULITES. Lam. A genus separated from Ammonites on account of the last volution covering the spire. This is generally considered as characterizing the Nautili, and distinguishing them from the Ammonites; but there are so many gradations that it seems impossible to maintain the distinction in this respect. Fig. 479, O. crassus, fig. 480, O. discus.

OREAS. Montf. Part of CRISTELLARIA, Lam. A genus of microscopic Foraminifera.

ORTHIS. Dalman. ([Greek: orthos], orthos, straight.) Fam. Brachiopoda, Lam. One of the generic divisions of Brachiopoda by Dalman, thus described: "Hinge rectilinear, with umbones distant; the larger valve with a transverse, basal, smooth area, with a triangular pit." O. basalis, fig. 207.

ORTHOCERA. Lam. See NODOSARIA.

ORTHOCERATA. Lam. A family of Polythalamous Cephalopoda, Lam. containing the following genera:--

1. CONULARIA. Conical, externally striated; no siphon. Fig. 469.

2. AMPLEXUS. Cylindrical; margins of the septa reflected. Fig. 463.

3. ORTHOCERATITES. Straight, gradually conical; septa simple; siphon central. Fig. 464.

4. NODOSARIA. Divided externally into lobes. Fig. 465.

5. BELEMNITES. Straight, conical; septa simple; siphon lateral; apex solid; internal cast, or nucleus, pyramidal, separable. Fig. 466 to 468.

6. CONILITES. Like Belemnites, but external shell thin at the apex. Fig. 470.

ORTHOCERATA. Bl. The first family of Polythalamacea, Bl. containing the genera Belemnites, Conularia, Conilites, Orthoceras and Baculites. De Blainville remarks that the genera included in this family are all fossils, and known very imperfectly, in consequence of the greater part of the specimens being only casts.

ORTHOCERATITES. Auct. Fam. Orthocerata, Lam. and Bl.--Descr. Straight, conical, divided into numerous chambers by simple septa perforated by a central siphon. O. annulata, fig. 464.

OSTEODESMA. Desh. PERIPLOMA, Schum.

OSTRACEA. (Ostracées, Lam.) A family belonging to the second section of the order Conchifera Monomyaria, the shells of which are described as irregular, foliaceous, sometimes papyraceous, with the ligament wholly or partly interior. The principal difference between the Ostracea and the Pectinides consists in the absence of the auricles and the foliated structure of the shells, for, although the Spondylus has ex-foliations or spines upon the external surface, the shell itself is compact and firm. This family contains the genera Gryphæa, Ostrea, Vulsella, Placuna, Anomia, which may be thus distinguished:--

1. PEDUM. Flat, turned up at the sides, an hiatus for the passage of a byssus. A triangular disc on the hinge. Fig. 179.

2. OSTREA. Foliaceous, irregular, hinge on a small triangular disc. Including Dendostrea, Ostræa, Exogyra, Gryphæa. Fig. 180 to 183.

3. PLACUNA. Two diverging ribs near the umbones. Fig. 184.