|Ostreidae, the true oysters, have very irregular shells, the shape of which is dependent on what it is attached to. The left shell is usually cemented to a hard substrate and is flatter than the right shell. The texture of the outer shell can be smooth to coarse, scaly or with growth rings. Inside, the shell is smooth, white, with a single purple or greenish muscle scar. (Most bivalves have 2 muscle scars on each shell.) The shells are usually encrusted with a variety of organisms, some of which bore into the shell for protection. The true oysters are different from the pearl oyster family, and though they can secret pearls, the results are usually irregular, small and of no value. Ostreidae are valued as food world-wide. There are 5 species found in Texas waters.