There are many variations and versions of the shell stitch. Here are two examples that really do look like shells.
The first shell is worked from side to side sharing the center hole. You may want to use your needle or laying tool to open that center hole so that your thread will pull through easier. Your last stitch is the long stitch in the middle. Don’t be afraid to vary the length and width of your shell to fill the space.
The second shell actually creates a shell shape. First you stitch your shell shape, again sharing that center hole. Beginning in the very center, needle weave through all the spokes, continuing until you run out of thread at the “foot” of the shell. Move up to the next spoke and continue to weave back and forth. When you get to the end of that spoke move up to the next spoke and continue weaving. You will have a row or two of weaving just at the top on the last three spokes. Really tight and even wraps will make this shell pop out and look like a shell.