Naticids make beveled holes, but muricid drill holes aremore straight-sided.  Borings presumably made by Ecphora in Pliocene Mercenaria, for example, are straight holes through the shell.
Boring organisms looking for space to live in rather than prey, such as polydorid worms, lithophagines, gastrochaenids, pholads, clionid sponges, certain barnacles and bryozoans, or certain sipunculids, may invade the shell of a living animal or a dead one.  Holes all the way through a shell without repair suggest post-mortem events, unless the hole is original (Diodora, Haliotis, Anomia, Thracia...).
Drilling near the umbo gets to most of the meat.
Some mantis shrimp can also make distinctive holes in shells.
Another feature of bivalves that has confused archaeologists is the fact that some bivalves open when cooked but close again when cooled.  Thus, a tightly closed arcid may have been eaten and not simply collected for ritual purposes.

Snails that resorb innner layers of the shell can easily be made into beads by grinding off the apex.  This is not a very useful approach for a predator, so olives, cones, etc. missing the tip are probably human modification.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
    [log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!