What is the relationship between snails/slugs and clams/mussels?
The Tree of Life web Mollusc page  shows that gastropods (snails/slugs/etc.) and bivalves (clams/mussels) are two of the major groups within Mollusca:



You can click through to get more detail on the individual groups (but there isn't much more information about the defining characters, date of divergence, etc.). At the very least, these technical names for the taxa (and of the Conchifera, their parent taxon) should help you search for more information, e.g. a Google scholar search on "bivalvia gastropoda divergence", which gets to Investigation of molluscan phylogeny using large-subunit and small-subunit nuclear rRNA sequences ("scaphopods are more closely related to gastropods and cephalopods than to bivalves") or Phylogenetic relationships of annelids, molluscs, and arthropods evidenced from molecules and morphology. (You do have to be careful in reading the phylogenetic literature, as it's rapidly evolving; the latter paper is from 1996 and may well be superseded.)