There are many types of bivalve fossils in Texas.  Oysters, Clams, Scallops, and other mollusks are all considered bivalves.  Some bivalves are symetrical, like clams, while others have different shaped sides, but most have a hinge that connects the two valves.  From Wikipedia: Bivalvia, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater mollusks that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. Bivalves have no head, and they also lack a radula. They include clamsoysterscocklesmusselsscallops, and numerous other families that live in saltwater, as well as a number of families that live in freshwater. The majority are filter feeders. The gills have evolved into ctenidia, specialized organs for feeding and breathing. Most bivalves bury themselves in sediment, where they are relatively safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. A few bore into wood, clay, or stone and live inside these substances. Some bivalves, such as the scallops, can swim. See the various types of fossils we find in Texas by clicking on a county below.

Check out Pelecypods in these counties:

Hood County

Cooke County

Denton County

Jack County

Kerr County