Bring a large pot of salted water to boil, gently place pasta shells in boiling water; bring water back to boil. Cook until noodles are just tender; drain well.
Squeeze spinach dry. Combine spinach, ricotta, 1/3 cup Parmesan cheese, fennel, basil and garlic in a large mixing bowl. Season with salt and pepper; mix well.
Spread 1/2 cup marinara sauce evenly over the bottom of a 9x13 inch baking dish.
Fill each pasta shell with the spinach cheese mixture. Arrange the shells, filling side up in the baking dish. Spoon remaining marinara sauce over the shells. Sprinkle remaining Parmesan cheese on top of the shells.
Cover the pan loosely with aluminum foil and bake in preheated oven until heated through; about 30 minutes.