Malleidae  Malleidae, or hammer oysters, is a family of saltwater clams. They are related to the pearl oysters, in the order Pteriida and the superfamily Pterioidea. The shells of most of these animals are T-shaped, with the hinge along the top of the T, and with the byssus emerging from the hinge. An oblique ligament holds the hinge. The shell is partially nacreous. There is a single, large adductor muscle. The exhalant current exits at the hinge. Most hammer oysters live in tropical, coralline areas. The following genera have been brought into synonymy.