The scallop is used to get the golden axe from Pascal. Pascal walks around by the beach at a random time, once a week. First, you need to buy a red turnip from Joan. Next, give it to Wendel the next time he comes to town. Wendel should give you the Turban or Country Guitar. If you get the Turban, take it to Sahara when she comes to town. Sahara should give you either the Massage Chair or Red Vase. If you get the Massage Chair, let Tortimer have it the next holiday and he should give you the scallop. Then just wait for Pascal to sho wup and give him the scallop.

But if you got the Country Guitar, give it to K.K. Slider to get K.K.'s Pic
And if you got the Red Vase, give it to Crazy Redd in exchange for the Safe. Give the safe to Tom Nook for the Tom Nook Pic.