Molluscan shell is a typical biocomposite in nature. It possesses excellent strength, stiffness and fracture toughness, which are closely related to its exquisite microstructure. SEM observation on clam’ shell shows that the shell is a kind of layered ceramic composite consisting of aragonite sheets and protein matrix. The aragonite sheets have long and thin shape and are arranged in a parallel manner. The higher fracture toughness of the shell is analyzed based on its representative model and the concept of maximal pullout energy. Analysis shows that the long and thin shape as well as the parallel arrangement of these sheets will substantially improve the maximal pullout energy of the sheets and the fracture toughness of the shell.