Piping a seashell can be tricky; they have so many tiny details that make them unique (and last I checked there isn't a piping tip marked "seashell"; you just make do with the same tips you use for everything else). The official state shell of North Carolina is the Scotch Bonnet (pronounced "bonay") and the only tips you'll need to pipe an approximation of it are round tips #12 and #5. You'll also need stiff consistency royal icing, a flower nail, parchment paper squares, a glue stick, a Styrofoam block to rest the nail in, and a dusting pouch filled with a 50/50 mix of powdered sugar and cornstarch. Start by sticking a parchment square to the flower nail with a dab from the glue stick.
AmeriColor or a small brush and some brown Color Dust to dab in a pattern of dots (in this photo you can see why the shell's name was inspired by Scottish tartan).