Part 54

4. PLACUNANOMIA. The same, but attached by fibres passing through a hole in one valve. Fig. 189 to 191.

5. ANOMIA. No costæ, attached by a bony substance passing through a hole in one valve. Fig. 186 to 188.

6. VULSELLA. Tongue-shaped, a ligamentary pit on the hinge. Fig. 185.

7. MULLERIA. Doubtful. Fig. 192.

OSTRACEA. Bl. The first family of the order Lamellibranchiata, Bl. containing the genera Anomia, Placuna, Harpax, Ostrea (including Dendostrea, Sw.) Gryphæa. To these may be added Placunanomia, Brod. and Mulleria.

OSTREA. Auct. ([Greek: ostreon], ostreon, a bone.) Fam. Ostracea, Lam. and Bl.--Descr. Irregular, inequivalve, generally inequilateral, foliaceous, attached by part of the lower valve; hinge sometimes slightly crenated; destitute of teeth; with the ligament spread upon the lower part of a central, triangular area, which is divided into three parts; upper valve much flatter than the lower; muscular impressions one in each valve, large, sub-central, sub-orbicular, with one very minute.--Obs. The Linnæan Genus Ostrea includes the Pectens and many other genera so different from each other that, without any desire to increase the number of genera, it was found necessary by subsequent authors to separate them. The common Oyster is the type of this genus as at present constituted, and is well known to be abundant in various parts of the world. Those which depart furthest from this type are the Gryphæa, Lam. with a prominent, incurved umbo in the lower valve. The Dendostrea, Sw. with margins characterized by strongly angulated folds, throws out arms from the lower valve, by which they are attached to stems of sea-weed, &c. Fig. 180, O. edulis. Fig. 181, O. folium. (Dendostrea, Sw.) Fig. 182, Gryphæa incurva. Fig. 183, Exogyra conica.

OTIDES. Bl. The first order of Scutibranchiata, Bl. containing the genera Haliotis and Ancylus.

OTION. Leach. ([Greek: ôtion], a little ear.) Order. Pedunculated Cirripedes, Lam.--Descr. Body sub-quadrate, supported on a fleshy pedicle with a gaping aperture and two posterior auricular tubes; valves five, separate, two semilunar, placed at the sides of the aperture, two terminal, very small, one dorsal, minute.--Obs. Otion differs from Cineras in having two cylindrical posterior tubes, and in the extreme minuteness of three out of five of the valves. Found on spars floating in the sea, &c. O. Cuvierii, (Lepas aurita, Linn.) Fig. 43, O. Cuvieri.

OTIS. Humph. AURICULA, Lam.

OVATE. (Ovatus.) Egg-shaped or oval.

OVEOLITHES. Montf. A microscopic shell resembling Bulla.

OVIPAROUS MOLLUSCA. Those which produce their young in eggs. Used in distinction from the VIVIPAROUS MOLLUSCA, whose young are perfectly formed before they leave the body of the parent.

OUTER LIP. See LABRUM.

OVULUM. Brug. (Ovum, an egg, dim.) Fam. Convoluta, Lam. Angyostomata, Bl.--Descr. Ovate or fusiform, smooth, convolute, spire covered; aperture narrow, with a canal at each extremity; outer lip crenulated, inflected; inner lip smooth, callous towards the spiral extremity; dorsal area wide, sometimes indistinctly marked.--Obs. The Ovula were placed by Linnæus in his genus Bulla, from which they are very remote. They differ from Cypræa in having the inner lip smooth. We have given representations of their different forms as follows: O. Ovum, fig. 442. O. verrucosum, (Calpurnus Montf.) fig. 441. O. Volva, the weaver's shuttle (Radius, Montf.) fig. 442. O. gibbosum, (Ultimus, Montf.) fig. 443.

OXYSTOMATA. Bl. The fifth family of Asiphonibranchiata, Bl. This family appears to have been formed for the express purpose of providing a place in the system for the genus Janthina, which seems to bear so little analogy with other genera of Mollusca, that conchological writers have been puzzled to know where to place it.

PACHYLABRA. Sw. PACHYSTOMA, Guild. A sub-genus of Ampullaria, the outer lip of which is thickened within. Ex. Ampullaria globosa.

PACHYMYA. Sow. ([Greek: pachus], pachus, thick, and Mya.) Fam. Cardiacea? Lam.--Descr. Obliquely elongated, equivalve, thick, sub-bilobed, with beaks near the anterior extremity; ligament partly immersed attached to prominent fulcra.--Obs. This singular fossil is shaped like Modiola, but the shell being extremely thick, and the ligament attached to a prominent fulcrum, it is difficult to know where to place it. Fig. 130, Pachymya Gigas.

PACHYSTOMA. Guild. ([Greek: pachus], pachus, thick; [Greek: stoma], stoma, mouth.) A genus composed of such species of Ampullaria, Auct. as have the edge of the aperture thickened and grooved within so as to form a sort of ledge upon which the operculum rests. Ampullaria globosa and corrugata are examples of this variation. The name Pachylabra is given to such species by Swainson, who objects to the above name on account of its having been previously used to a genus of fishes. Fig. 539.

PACHYTOMA. Sw. A sub-genus of Helicina, thus described, "Aperture entire; the inner lip very thick; the spiral whorls hardly convex; P. occidentalis. Zool. J. iii. 15. f. 6-10. viridis, Zool. Journ. i. pl. 6. f. 7." Sw. p. 337.

PACLITES. Montf. A genus composed of species of Belemnites, Auct. described towards the extremity, with a pore, at the apex, and a straight lengthened aperture. Ex. B. ungulatus, Bl.

PADOLLUS. Montf. A genus composed of species of HALIOTIS, with a strongly marked spiral groove. Ex. H. tricostalis, Lam. Fig. 339.

PAGODELLA. Sw. A sub-genus of Trochus, thus described: "Trochiform; generally thin, and always not pearlaceous; aperture and pillar perfectly united and entire; operculum horny. P. major. Mart. 163. f. 1541, 1542. tectum-persicum. Ib. f. 1543, 1544." Sw. p. 351.

PALLEAL IMPRESSION. (Pallium, a mantle.) The mark or groove formed in a bivalve shell by the muscular attachment of the mantle, which, being always found near the margin of the shell, is sometimes termed the marginal impression. In bivalves with two muscular impressions it passes from one to the other. If in passing, it takes a bend inwards posteriorly, it is said to be sinuated, and that part is called by Mr. Gray, the Siphonal scar.

PALLIOBRANCHIATA. Bl. The first order of the class Acephalophora. Bl. The animals of this order are described as more or less compressed, included between the two valves of a bivalve shell, one inferior, the other superior, joining at the back and opening in front. The Palliobranchiata in the system of De Blainville correspond with the Brachiopoda in the system of Lamarck, and the shells may be known by their being symmetrical. This order contains in the first section of symmetrical bivalves, Lingula, Terebratula, Thecidium, Strophomena, Plagiostoma, Dianchora and Podopsis: in the second section, Orbicula and Crania.

PALMATED. Flattened like a palm, as the fronds or fringes of some Murices.

PALMINA. Gray. Differing from OTION in having but one auricle.

PALUDINA. Lam. Fam. Peristomata, Lam. Cricostomata, Bl--Descr. Varying in form from oval to globose, in some instances oblong, covered with a greenish horny epidermis; spire acute, composed of rounded whorls; aperture ovate; peritreme entire, slightly modified by the last whorl; operculum horny, concentric. Europe, North America, East Indies, China, &c.--Obs. The construction of the operculum distinguishes this genus of freshwater shells from Valvata and Cyclostoma. The Paludinæ are viviparous. Fig. 321. P. Achatina.

PALUDOMUS. Sw. A genus of the family of "Melanianæ," Sw. described as differing from Melania in having the spire shorter than the aperture. Sw. p. 340.