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As the name implies, bivalves comprise two shells protecting their soft body, hinged with a ligament to enable feeding, respiration and burrowing. They are very common members of modern shallow water marine environments as well as fresh and brackish waters.
They are readily distiguished from another two valved group (in a completely different phylum), the brachiopods, by their between valve line of symmetry.
Two atypical bivalves, the oysters and the rudists, are represented in large numbers in this repository. Oysters still exist today although many of those evident in the Cretaceous rocks of Central Texas, such as the genus Exogyra, are now extinct.
Rudists evolved in the late Jurassic and became extinct at the end of the Cretaceous.
BEG34108 Trigonia clavigera
Cretaceous, Weno Formation
Cooke County, Texas
This particular clam is often preserved as impressions of its outer shell. These enhance the natural beauty of the famous 'Cordova shell' building stone quarried to the north of Austin.