This beveled and circular borehole was made in a Pacific Littleneck Clam (Leukoma staminea).
And the animal responsible for the borehole?
Lewis's Moon Snail (Neverita lewisii). Moon snails have an accessory boring organ (ABO) and can secrete an acidic substance that softens bivalve shells. After securing their prey with a large muscular foot, they'll alternate applying acid (with the ABO) and rasping away the softened shell with their file-like radula until they break through. The snails then release digestive enzymes to soften the bivalve's tissues and feed on the bivalve through the borehole.
Moon snails are remarkable predators that leave definitive evidence of their past feeding activity.
P.S. If you missed them before (or just want to enjoy them again!), I posted some nice microscope pictures of a moon snail on 5 August 2013.