By using X-ray diffractometry, the development of preferred orientation was studied in the calcitic eggshells of the Crocodylia and the aragonitic eggshells of the Testudines. In all species the texture that develops is one in which the basal plane of the unit cell in each of the two crystal systems tends to lie parallel to the shell surface. Preferred orientation reaches a maximum at the exterior surface and its development through the shell is normally uninterrupted. In some species of the Testudines, the interior surface shows a radial direction of crystal growth from the nucleation centres which is at right angles to the (001) plane. In others, there is a small degree of (001) preferred orientation at the interior surface as a result of inward growth from the nucleation centres. The aragonite of which the turtle shells are composed shows peak intensity values that differ markedly from those of synthetic aragonite and yet are common to all the families of the Testudines studied. Moreover, the aragonite is unusually stable in that it shows no partial inversion to calcite after grinding. The method of analysis for demonstrating the development of preferred orientation has proved to be equally as effective when applied to aragonite as in its previous use for analysis of calcite.