Molluscan shell possesses excellent strength, stiffness and fracture toughness that are closely related to its exquisite microstructure. SEM observation of a clam’ shell showed that the shell is a kind of bioceramic composite consisting of aragonite and protein layers parallel with the surface of the shell. The observation also showed that the aragonite layers are composed of long and thin aragonite sheets. Many aragonite sheets are of curving shape at the center of the shell. The higher fracture toughness of the shell was analyzed based on the representative model of the curving aragonite sheets and the concept of the maximum pullout force that is related to the fracture toughness of the shell. The analytical result showed that the maximum pullout force of the curving aragonite sheet is larger than that of straight aragonite sheets, which may effectively enhance the fracture toughness of the shell.