I'd lean towards a gastropod myself. Something with a distinctive outline,
maybe a spider conch? Bear in mind that tats blur over the years, though you
can always get them touched up. I'd pick the artwork before I got to the
parlor, instaed of chosing flash or letting the tatoo artist interpret a
photo or a shell. If you can find a good line drawing of a species you like,
use that.

It's not a mollusk, but my girlfriend has a totally awesome tat of a
brittlestar. The artwork is from Gustave Haekels' "Artforms in Nature". (I
may have misspelled the name, can't seem to lay my hands on the book just
now. But that dude could draw.) Check out line art in old books and
reproductions. I'll bet you'll find something way cool.