Which bivalve subclass includes cockles, clams, shipworms, and giant clams?

The bivalve subclass that includes cockles, clams, shipworms, and giant clams is the Heterodonta.1

While classification systems for Bivalvia can vary among experts, the subclass Heterodonta is generally characterized by highly variable shell forms and hinge teeth that may be reduced or absent. Members of this group typically possess eulamellibranch ctenidia and extensively fused mantle margins, which often form long siphons. Within this subclass, the order Veneroida includes a wide variety of clams and cockles, while the order Myoida includes shipworms.213