The eggshells of 56 chelonians were examined by electron microscopy and X-ray diffractometry. They were classified into six types in terms of the matrix structure of their calcareous layer; type I was composed of a thin calcareous layer with minerals in an amorphous structure; type II with shell units composed of mammillary cores calcified with aragonite crystals; type III with shell units composed of mammillary cores, plus a single palisade layer also calcified with aragonite crystals, and with each shell unit separated; type IV with shell units the same as type III, but tightly packed together; type V with shell units composed of mammillary cores plus two palisade layers; and type VI with a cuticle layer calcified with calcite crystals over the same structure as that of type V. X-ray diffraction analyses at the outer surface of eggshells showed a gradual change in crystal disposition from the random disposition of type II to the single direction-oriented disposition of type V. The shell height was approximately parallel to the development of the palisade-layer matrix. The limiting membrane of all eggshell types was perforated with canals and that of type I was partially missing. Type I had a parchment shell, types II and III had a pliable shell (some were rigid) and types IV to VI had rigid shells. The present study showed that the hardness of eggshells can be determined by the composition of the shell matrices, as shell matrices are the framework for mineralization.