Spear scallop

The shell of the spiny scallop is slightly shaped like a fan and is able to grow to a height of about 9 centimetres (3.5 in) though a more normal adult size is 6 centimetres (2.4 in). The shell is composed of two valves, each of which is convex and has a small number of broad ribs covered with blunt spines. These radiate from the umbone, the rounded protuberance near the hinge, and between them are fine etched striations. The background colour is white with radial bands of pale purple and the right valve, which is usually underneath, is paler than the left. The annual growth rings are visible, often as concentric bands of a different hue. Beside the hinge are two irregular shell flaps or auricles with the anterior one normally being much larger than the other. This provides an attachment for the single strong adductor muscle that closes the shell. On either side of the long hinge there are some little ridges and grooves known as teeth. Their function is to prevent the valves moving sideways with regard to each other. Some bivalve shells have large "cardinal" teeth on the hinge immediately below the umbone, but the spiny scallop does not. Instead it has 5 or 6 lateral teeth lying on either side of the hinge. Lining the inside of the valves is the mantle, a membrane that covers the gut and other internal organs. It can be seen round the margins of the valves as a thick layer like a sandwich filling. It is fringed with numerous short tentacles and there is a row of tiny simple eyes close to each of the valves. The animal usually lies on its right valve and its exposed, left valve often has a colonial sponge growing on it. The spiny scallop can be distinguished from its close relative the Pacific pink scallop (Chlamys rubida) by the valves being less rounded and by the small curved spines on the ribs which give it a rough texture whereas the Pacific pink feels smooth. The glossy white interior of the spiny scallop's shell often has a purplish blotch near the umbone, not present in the Pacific pink.