OSTRZOID.E, a family of lamellibranchiate mollusks of which the common oyster (ostrea) is the type. It is sometimes made to include another group, the pectinidce, com prising the scallops, and is sometimes named pectinidte, instead of ostrwidre. The following are the chief characteristics of the ostraidce: Shell inequivalve, slightly ine quirateral, free or attached to-the bottom or some object; hinge usually toothless, liga ment internal; lobes of the mantle entirely separated; foot small or wanting; a single adductor muscle. In the genus ostrea the shell is irregular and attached by the left valve, which is convex, and has a well-marked beak. The upper valve is usually concave or flat, and is the smaller. The hinge is toothless, having a single ligamental pit. The lower valve is more plaited than the upper, and both valves are in some species thus ornamented, as.iu ostrea Marshii of the oolitic formation. In the genus gryphma the lower valve is much the longer and very much curved, so that it points inwards, and at right angles to the plaue of the upper valve, the whole animal having a general resem blance in profile to an ammonite, although belonging to an entirely different class of mollusks. They existed during the triassic and cretaceous epochs. In the genus exogyra, which existed in the later oolitic and cretaceous epochs, the beaks are reversed, or turned toward the posterior side of the shell. The genus ostrea Commences in the carboniferous seas, abounds in the secondary and tertiary periods, and is very plentiful at the present day. A genus, periwstra, has been founded for Jurassic oysters, which differ from ostrea proper in having the ligament contained in from four to eight transverse grooves or pits. In a second section or sub-family, anomiac.?a,, of ostrimidte there are various forms of which anemia may he taken as the type. This latter genus comprises bivalves having a very thin, translucent shell, which is attached to some body by a plug which passes through a hole or notch in the right valve. It appears to have commenced its existence
in the cretaceous period, but limanomia of Devonian, anomianella of the carboniferous, and pbtcunopsis of the Jurassic are.older forms of the same type. The genus placana, commencing in the tertiary, is allied in many respects to anontia, but the shell is unat tached. For the group peetinidm see PECTEN, ante. A fourth group of ostrwidm, some times regarded as a distinct family (limada), is represented by the typical genus lima, the shell of which is equivalve and unattached, and the beaks separated from one another and eared. The surface is usually partially covered with radiating ribs or ridges, and there is a median cartilage pit and a triangular hinge area. The genus appears to have commenced in the carboniferous, was abundant in the triassic, Jurassic, cretaceous, and tertiary formations, and has representatives at the present day in smaller numbers. The limoptem of the Devonian is perhaps an early representative. A fifth group of ostrmidm, which is sometimes ranked as a separate family (spondylida') is represented by what are known as the thorny oysters, and forms the genus spondylus. It has an inequivalve shell, and is attached by the right valve to some other body. The beaks are separated from each other, and the shell is covered with spines or thorns, inclined direction from the beak. The genus commenced in the Jurassic. was plentiful in the cretaceous, and has present representatives. The plicatake resemble spondylus. by having an inequi valve shell which is attached by the right valve, and by having two binge teeth An each valve. But the shell is seldom eared, the hinge area is obscure, and the valves arc not armed with spines. They extend in time from the tries to the present epoch, and were exceedingly abundant in the lower greeusand. The anatomy of the oyster is given in the article under that title.