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.