1. Mytilus. Umbones terminating in a point. Fig. 158.
2. Dreissina. The same, with a septiform plate. Fig. 159.
3. Modiola. Anterior margin rounded beyond the umbones. Fig. 160.
4. Pinna. Open at the posterior extremity. Fig. 162.
5. Lithodomus. Cylindrical, living in holes. Fig. 161.
MYTILUS. Auct. Fam. Mytilacea. Lam.—Descr. Equivalve, cuneiform, oblique, smooth, with umbones terminal, pointed, and posterior side broad, rounded; hinge linear, with a long, partly internal ligament; muscular impressions two in each valve, that on the posterior side large, irregular; that on the anterior small; palleal impression irregular.—Obs. The Linnean genus Mytilus included the Modiolæ, which differ from the Mytili in the rounded anterior side; and the Pinnæ, which are large shells, gaping at the posterior extremity. M. achatinus, fig. 158.
NÆARA. Gray. A genus composed of Anatina longirostrum, Lam. and other similar species.
NAIA. Sw. A sub-genus of Castalia, Lam. thus described: "Oval, cardinal teeth beneath the bosses, and deeply sulcated, C. corrugata. Lam. En. Méth. 248. f. 8, picta. Sw. En. Méth. 248. f. 6." Sw. p. 379.
NANINIA. Gray. A genus composed of the planorbicular species of Helix, with large umbilici, and outer lip thin, included in the sub-genus Helicella, Fer. Ex. H. citrina, fig. 280.
NASSA. Lam. A genus of small shells united to Buccinum by some authors, but separated by others on account of the little tooth-like projection terminating the columella. N. arcularia, fig. 423.
NATICA. Brug. Fam. Neritacea, Lam. Hemicyclostomata, Bl.—Descr. Globose, thick, generally smooth; spire short, pointed, with few volutions; aperture semilunar, entire; outer lip thin; columellar lip oblique, nearly straight, callous; umbilicus with a spiral callosity, terminating behind the columella, and sometimes filling up the cavity; operculum shelly in some species, horny in others; epidermis thin, light, semitransparent.—Obs. The straight, callous, smooth edge of the columella and the callosity serve to distinguish this genus from Nerita, Neritina, Neritopsis and Helix. Fig. 327, 328.
NATICARIA. Sw. A sub-genus of Natica, thus described: "Oval; convex above; umbilicus small, open, placed very near the top of the aperture; inner lip reflected; small. N. melanostoma, Mart. 189. f. 1926, 1927. cancellata, Sw. Ib. 189. f. 1939. bifasciata, Griff. Cuv. 1. f. 2." Sw. p. 346.
NATICELLA. Guild. A sub-genus of Natica, thus described: "Operculum horny; shell globose, but generally depressed; umbilicus nearly filled up by a vitreous deposition of the inner lip; spire obtuse. N. aurantia. Mart. 189. f. 1934, 1935." Sw. p. 345.
NAVICELLA. Lam. (A little ship.) Fam. Neritacea, Lam. Hemicyclostomata, Bl.—Descr. Transversely oval, symmetrical, smooth; aperture entire, oval; dorsal surface convex; outer lip thin; inner lip flat, straight edged; spread over the front surface of the body whorl, and sometimes hiding the apex; apex incurved; operculum testaceous, flat, sub-quadrate, with a lateral articulation.—Obs. This well known genus, of which there are several species, is named Cimber by Montfort. The shells are brought from India, the Isle of France and the Moluccas. Fig. 323, N. elliptica.
NAUTELLIPSITES. Parkinson. A generic name proposed to include such species of Nautilus as have been compressed, so as to assume an oval instead of a round form. The genus Ellipsolites of De Montfort consists of species of Ammonites similarly deformed.
NAUTILACEA. Bl. The fifth family of Polythalamacea, Bl. the shells of which are described as more or less discoidal, compressed, symmetrically convolute; the last whorl much longer than the others; which are entirely hidden beneath it and advancing beyond the last but one, so as constantly to form a large oval aperture, which is always, however, modified by the last whorl. The septa are united in the greater number of instances and pierced by one or more (?) siphons. This family contains the genera Orbulites, Nautilus, Polystomella and Lenticulina.
NAUTILACEA. Lam. The sixth family of Polythalamous Cephalopoda, Lam. containing the genera Discorbites, Siderolites, Polystomella, Vorticialis, Nummulites, Nautilus. To these may be added Simplegas and Endosiphonites. Fig. 472 to 476.
NAUTILUS. Auct. (A little boat.) Fam. Nautilacea, Lam. and Bl.—Descr. Convolute, discoid, chambered, symmetrical; spire partly or entirely concealed by the last whorl; aperture modified by the last whorl, wide, sinuated on the dorsal margin; interior surface pearly; septa dividing the chambers simple; siphon discontinuous.—Obs. The shell named Nautilus by Pliny is the Argonauta of modern authors, a thin shell, not chambered. The Nautili are known from the Ammonites by the septa being simple, not sinuated as in the latter genus, and in general the volutions of the spire are not visible. Three or four species are known inhabitants of the Pacific Ocean and Australian Ocean. The fossil species are found in the tertiary, and also in the secondary strata, as low down as the Mountain limestone. N. pompilius, Frontispiece.
NAYADES. Lam. A family of the order Conchifera Dimyaria, Lam. described as containing fresh-water bivalve shells, with or without teeth on the hinge. They are all pearly within, and have a thick, rather smooth epidermis without. This family contains a great variety of shells, which have been separated into an immense number of genera, but which G. B. Sowerby, sen. gives very good reasons for uniting under one generic name. The most generally received distinctions are as follows:
1. Castalia. Two cardinal, one lateral, ribbed teeth. This genus is removed from the family of Trigonacea. Fig. 140.
2. Unio. Teeth various. Fig. 142, 145, 149, 148, 147, 151, 141.
3. Hyria. Trigonal, alated. Fig. 143, 150.
4. Anodon. No teeth. Fig. 152.
5. Iridina. Hinge crenated. Fig. 150.
NECTOPODA. Bl. The first family Nucleobranchiata, Bl. containing the genera Carinaria and Firola; the latter is not a shell.
NEMATOPODA. Bl. The first class of the sub-type Malentozoa, Bl. containing all the mollusca with multivalve shells, except Chiton, and divided into the families Lepadicea and Balanidea, corresponding with Lamarck's sessile and pedunculated Cirripedes, and with the Linnæan genus Lepas.
NEMATURA. Benson. Fam. Turbinacea, Lam.—Descr. Thin, nearly oval, somewhat compressed from back to front; spire acute, consisting of few rounded whorls; last whorl large, but contracted near the aperture; aperture small, oblique, rounded anteriorly; peritreme continuous, thin; operculum spiral, horny, with few volutions.—Obs. The distinguishing character of this genus is the contraction of the last whorl near the aperture, in which respect it is nearly resembled by the shell called Cyclostoma lucidum. Two recent and one fossil species, all very minute, are described by Sowerby in Loudon's Magazine of Natural History, New Series. Fig. 305.
NERINEA. Defr. Fam. Canalifera, Lam.—Descr. Turrited, oblong, sub-canaliculated, consisting of numerous whorls; aperture with a strong fold on the columella, one on the outer lip, and one on the inner lip at the edge of the body whorl.—Obs. This genus is only found in a fossil state usually in the Oolitic beds, it is not resembled by any other; the strong, prominent folds on the three upper angles of the subquadrate aperture present a singular appearance in a section. One species has been named N. Hieroglyphus. We give N. Goodhallii, fig. 374.
NERITA. Auct. Fam. Neritacea, Lam. Hemicyclostomata, Bl.—Descr. Smooth or ribbed, semiglobose; spire short, sometimes flat, consisting of few volutions; aperture large, semilunar; outer lip thick, entire; inner lip thickened, dentated at the edge, spread over the body whorl, forming a flattened disc; operculum shelly, spiral, with an appendage by which it is locked under the sharp edge of the columella.—Obs. These marine shells are known from Neritina by the thickness of the shell and the want of the thick, horny, dark coloured epidermis; from Natica by the flat area produced by the spreading of the thickened columellar lip. N. Peloronta, fig. 330. N. polita, fig. 329.
NERITACEA. Lam. A family of the first order of Trachelipoda, Lam. containing the following genera:
1. Navicella. Apex terminal, not spiral; inner lip septiform. Fig. 323.
2. Nerita. Columellar lip septiform, edge with distinct teeth; shell thick. Fig. 330.
3. Neritina. Shell thin; columellar lip septiform, edge denticulated; generally a thick, dark coloured epidermis. Fig. 324 to 326.
4. Natica. Having an umbilicus behind the columellar lip, with a spiral callosity. Fig. 327, 328.
5. Neritopsis. Edge of the columellar lip with a deep notch. Fig. 331.
6. Pileolus. Patelliform; apex central; columellar lip septiform, leaving the aperture small. Fig. 332.
7. Janthina. Columellar lip linear; aperture angulated. Fig. 333.
NERITINA. Lam. Fam. Neritacea, Lam.—Descr. Thin, semiglobose, obliquely oval, smooth, flattish in front; spire short, sometimes depressed, consisting of few rapidly increasing whorls; aperture semicircular; outer lip thin, sharp; columellar lip broad, flat, its inner edge straight, denticulated; operculum testaceous, semicircular, sub-spiral, with an articulating process on the inner edge.—Obs. This genus of fresh-water shells differs from Nerita in the minuteness of the denticulation of the columella, as well as in the characters mentioned in our observations upon the latter genus. N. spinosa, (Clithon, Montf.) fig. 325. N. virginea, fig. 324. N. perversa, Lam. (Velates, Montf.) fig. 326. All the species known up to the present time, with the exception of three, are represented in the author's Conchological Illustrations, parts 86, 87, 90, 91, 94 to 100. The catalogue accompanying these representations enumerates 59 species.
NERITOPSIS. Gray. Fam. Neritacea, Lam.—Descr. Sub-globose, thick, cancellated; spire short, composed of few rapidly increasing whorls; aperture transverse, sub-orbicular; outer lip thickened within; columellar lip thick, rather flat, with a large rounded notch in the centre of its inner edge.—Obs. This genus most nearly resembles Nerita, from which it differs in the peculiar notch of the columella. N. granosa, fig. 331.
NICANIA. Leach. Astarte, Sowerby. The same as Crassina of Lamarck.
NITIDELLA. Sw. A genus of "Columbellinæ," Sw. thus described: "Bucciniform, small, ovate, smooth, glassy; aperture effuse; outer lip slightly thickened, faintly inflexed, and generally striated internally; inner lip somewhat flattened above; base of the pillar with one or two slight internal folds, or a single angular projection. Columbella nitida, Lam. (fig. 17, c. p. 151.)" Sw. p. 313.
NOBIA. Leach. Order, Sessile Cirripedes, Lam. This genus resembles Pyrgoma, Auct. consisting of a conical paries, supported upon a funnel-shaped cavity in the madrepore, but differs in its operculum, which consists of two valves, whereas that of Pyrgoma has four. N. grandis, fig. 29.
NODOSARIA. Lam. and Orthocera have been united by Sowerby under the name of the first. Fam. Orthocerata, Lam. and Bl.—Descr. Straight, chambered, elongated; chambers more or less ventricose; septa perforated by a central siphon.—Obs. This genus consists only of fossils found in sub-appenine tertiary beds. It is placed by De Blainville in one of his divisions of the genus Orthoceras, which is characterized as "species not striated, and with chambers very much inflated." N. æqualis, fig. 465.
NODOSE. Having tubercles or knobs.
NOGROBS. Montf. A fossil appearing from the figure and description to resemble Belemnites.
NONION. Montf. A genus of microscopic Foraminifera.
NONIONINA. D'Orb. A genus of microscopic Foraminifera.
NOTREMA. Rafinesque. A shell described as composed of three integral valves, concerning which De Blainville puts the query, "ne seroit-ce pas plutôt une Balanide mal observée?"
NOVACULINA. Benson. (Novacula, a razor.) Fam. Solenacea, Lam.—Descr. Equivalve, inequilateral, transversely elongated; external ligament communicating with the interior of the shell by an oblique channel; beaks prominent; hinge line nearly straight, with one narrow curved cardinal tooth in one valve, entering between two similar teeth in the other; siphonal scar long; extremities of the shell gaping; epidermis thin, light brown, folding over the edges and connecting the dorsal margins. Hab. Jumna, Gooti, and Ganges. Fig. 63.
NUCLEOBRANCHIATA. Bl. The fifth order of the second section of Paracephalophora Monoica, Bl. the shells of which are described as symmetrical, more or less curved, or longitudinally rolled up and very thin. This order contains, Fam. 1. Nectopoda, containing Carinaria; Fam. 2. Pteropoda, containing Atlanta, Spiratella and Argonauta.
NUCLEUS. (A kernel.) Anything forming a centre around which matter is gathered. The nucleus of shells is the first formed part; the first deposit of shelly matter to which the successive layers are added; the apex of the spiral cone, of which most shells are composed. (See Cone.) The nucleus is formed within the egg in oviparous, and within the old shell in viviparous mollusca. It is frequently more transparent and light than the remainder of the shell, and sometimes falls off; when this occurs the shell is said to be decollated.
NUCULA. Lam. (A small nut.) Fam. Arcacea, Bl. and Lam.—Descr. Equivalve, inequilateral, transverse, covered with an epidermis; hinge linear, with a series of sharp, angulated teeth, arranged in a line on each side of the umbones, and a central ligamentary pit; muscular impressions two, simple; palleal impressions not sinuated.—Obs. The row of teeth on each side of the umbones, and the ligamentary pit in the centre of the hinge prevent the pretty little shells of this genus from being confounded with any other. Thirty-four figures are enumerated in the catalogue by Sowerby, sen. which accompanies the Conchological Illustrations of the author. The new species, to the amount of 24, have been figured in parts 14 to 16, of the above mentioned work. Recent Nuculæ are found from the frozen to the torrid zones, and the fossil species occur in nearly all the beds from the Pliocene to the Carboniferous system.
NUMMULACEA. Bl. The third family of Cellulacea, Bl. described as containing shells or calcareous bodies, which are characterized as discoidal, lenticular; without the slightest traces of whorls to be seen externally. The whorls are numerous, internal, and divided into a great number of cells, which are separated from each other by imperforate septa. This family contains the genera Nummulites, Siderolites, Vorticialis, Helicites, Orbiculina, Placentula.
NUMMULTTES. Lam. (Nummus, money.) Fam. Nautilacea, Lam.—Descr. Orbicular, convolute, shewing no trace of spire externally; interior divided into cells spirally arranged.—Obs. The singular fossils composing this genus receive their name from their external resemblance to a battered coin. Fig. 472. N. lenticulina.
NUX. Humph. Cyclas, Lam.
NYMPHACEA. Lam. A family belonging to the order Conchifera Dimyaria, Lam. Ligament external, placed on a prominent fulcrum. This family contains the following genera:
1. Sanguinolaria. Rostrated, gaping; two cardinal teeth in each valve, including Soletellina and Lobaria. Fig. 98, 99.
2. Psammobia. Quadrate; valves closed, including Psammotæa. Fig. 100.
3. Corbis. Thick, fimbriated; a cardinal tooth in the centre of a pit. Fig. 101.
4. Grateloupia. A series of small teeth filling a triangular area. Fig. 102.
5. Egeria. One single and one double cardinal tooth. Fig. 103.
6. Lucina. Rounded; anterior muscular impression tongue-shaped. Fig. 104.
7. Tellina. An anterior fold in the ventral margin; lateral teeth. Fig. 105, 106.
8. Tellinides. No anterior fold; no lateral teeth. Fig. 107.
9. Donax. Margin denticulated; shell wedge-shaped. Fig. 108.
10. Capsa. Margin not denticulated, no lateral teeth. Fig. 109.
OBELISCUS. Humph. Trochus, Lam.
OBLIQUE. (obliquus. lat.) In a slanting direction. The whorls of spiral univalves generally take an oblique direction in reference to the imaginary axis of the shell. A bivalve is said to be oblique when it slants off from the umbones. An example of this is seen in Avicula, fig. 163.
OBSOLETE. (obsoletus, lat.) Worn out, out of use. This term is used to express an indistinctness of character, which sometimes results from the action of sea-water upon unprotected parts of the shell, and sometimes from the deposits of enamel formed in age, and covering the early striæ, ribs, teeth, &c. thereby rendering them less acute.
OBTUSE. (obtusus, blunt.) The application of this term is not peculiar to conchology. It is most frequently used to express the character of the spire. Ex. The apex of Megaspira, fig. 294.
OCEANUS. Montf. ("Corne d'ammon vivant," Fr.) Nautilus umbilicatus, Auct.
OCTHOSIA. Ranz. Clitia, Leach.
OCTOCERA. Bl. The first family of the order Cryptodibranchiata, Bl. containing the genus Octopus. A species of which being found in the Argonauta, or Paper Sailor, has given rise to the long continued controversy as to whether it is really the constructor of the shell, or whether it is a mere pirate, and having destroyed the true animal of the Argonaut, has possessed itself of the habitation. This question is now set at rest. See Argonauta.
OCTOGONAL. (octogonum.) Having eight angles. For an example, see Dentalium, fig. 2.
OCTOMERIS. Sow. (οκτω, octo, eight; μερος, meros, part.) Fam. Balanidea, Bl. Order, Sessile Cirripedes, Lam.—Descr. Eight principal valves circularly arranged, forming a compressed cone, attached by a jagged base; aperture enclosed by an operculum, consisting of four valves in pairs.—Obs. The only genus of Sessile Cirripedes agreeing with this in the number of principal valves is Catophragmus, Sow. which is, however, sufficiently distinguished by the several rows of smaller valves by which the principals are surrounded at the base. O. angulosus, fig. 24.
ODONTIS. Sow. Monodonta, Lam.
ODOSTOMIA. Flem. Descr. "Shell conical; aperture ovate; peristome incomplete, retrally, and furnished with a tooth on the pillar." A genus composed of several small species of land shells. Turbo plicatus, Spiralis, Unidentatus, &c. Mont.
OLIVA. Auct. (An olive.) Fam. Convoluta, Lam. Angyostomata. Bl.—Descr. Oblong, cylindrical, thick, smooth, shining; spire very short, with sutures distinct, aperture elongated, notched at both extremities; outer lip generally thick; columella thick, obliquely striated, terminated by a tumid, oblique, striated varix; a raised band passing round the lower part of the body whorl.—Obs. The shells composing this well known genus present a great variety of rich markings and brilliant colours. They are marine and tropical. Fossil species are found sparingly in the London Clay and Calcaire-grossièr. The Ancillariæ are distinguished from this genus by the sutures of the whorls being covered by enamel. O. maura, fig. 457.
OLIVELLA. Sw. A genus of "Olivinæ," Sw. thus described: "Oliviform; spire (typically) rather produced; the tip acute; inner lip not thickened; outer lip straight; base of the pillar curved inwards, and marked by two strong plaits; upper plaits obsolete or wanting; aperture effused at the base only; biplicata, Tank. Cat. 2332. purpurata. Zool. Ill. ii. 58. f. 1. mutabilis. Say. eburnea. Zool. Ill. ii. 58, f. 2. conoidalis. Lam. No. 57. oryza. Lam. No. 62."
OLYGYRA. Say. Mentioned by Ranz as properly belonging to Helicina. H. neritella, Auct.
OMALAXIS. Desh. Subsequently Bifrontia. Desh. Fig. 354.
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 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. (ορθος, 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:—