Let's think about the riddle step by step:
Clue 1: "With a shell, it crawls slowly on the sand." This tells us the animal has a shell and moves slowly on sand.
Clue 2: "Its flippers are made for the sea, not the land." This means it has flippers specifically for swimming in the sea, not for moving fast on the land.
Putting these clues together, the animal is one that lives near the sea, has a shell, crawls on the sand, and swims with flippers.
The answer is: a sea turtle.