|Mactridae, the surf clams, have shells that are oval, triangular or oblong. The outer shell may be smooth or have concentric ridges. Many have a rounded ridge on its posterior end. In the beak of the shells is an oval pocket (chondrophore) which holds the internal ligaments. The surf clams burrow in sandy or muddy bottoms. Many are very common and tend to dominate, in numbers, the areas where they are found. Many are of commercial importance. Some, like the Rangia shells, are commonly used as road-bed material. There are 7 species found in Texas waters.