The obvious commonality among all bivalves is that they have two shells joined together by strong ligaments. The side with the ligaments are called the dorsal margin in contrast to the ventral margin. The bivalves may attach themselves to a hard surface, hide under rocks or in crevices. Many live buried in mud or sand.
Bivalves constitute a class 30000 species of freshwater and marine mollusks, better known as mussels, scallops, clams and oysters. The following orders are well represented along the coast of Norway:
- File shells (Limoida)
- Soft shell calms (Myoida)
- True mussels (Mytiloida)
- Oysters and scallops(Ostreoida)
- Hard shell calms and cockles (Veneroida)
- Rasor shells are recently moved from Veneroida to a yet not formally accepted order, Euheterodonta.