It would be nice of there was a single hierarchy that everybody agreed upon, but that is not the case. Because science progresses (usually!), different experts see things differently, resulting in multiple hierarchies (multiple ways of grouping creatures together in terms of how similar they are to each other). That applies within bivalves. If you go here - http://en.wikipedia.org/wiki/Bivalvia, and serch for 'Bivalvian taxonomies', you'll see what I mean. But, one way or another, you'll get down to species. For instance, this page shows the species in the genus Mya - http://en.wikipedia.org/wiki/Myidae.
You asked a simple question. Perhaps I should apologise for giving what appears to be a complicated answer. But, to some extent, that is just the way it is.