A scanning electron microscope (SEM) was used for observing the microstructures of a Mactridae shell. It showed that the shell is a kind of natural bioceramic composite, which consists of aragonite sheets and organic matrix with laminated structure. It also showed that there are various reinforced microstructures in the shell, which include a kind of lambdoidal one. The maximum pullout force of the lambdoidal reinforced microstructure, which is related to the fracture toughness of the shell, was analyzed and compared with that of a conventional 0°-structure based on their representative models. The result indicated that the maximum pullout force of the lambdoidal reinforced microstructure is markedly larger than that of the 0°-structure, which was experimentally verified.