The Ace Hotel in New York City had an exhibit called, "The Smallest Mollusk," and it reminded me that I once found an impossibly tiny snail shell, which is of course a member of the taxonomic class Gastropoda in the phylum Mollusca. (Thanks, Google.)
More, in case the link above goes away https://micro.ooo/mollusk
I took these photos in July 2003, when I had what was then a pretty nice digital camera, but still, I didn't know how to use the macro focus, so... the smallest mollusk is also kind of blurry. It's clear enough though that it is barely bigger than the head of a thumb tack and fits neatly between the lines on a piece of notebook paper.