The study of the morphogenesis of bony plates in Trionyx sinensis showed, that no independent epithecal ossifications are present over the bony anlages at all the stages of ontogenesis. Thus, the plates of Trionychidae are homologous to these in other turtles. The morphological differences between internal rod-shaped and external callosal parts of bony elements are predetermined by the stratification of primary derma into the histologically different strata - hypodermal, fibrous and papillary ones. Two stages can be distinguished in the development of the bony structures in the armour of trionychids: the first stage (plesiomorphic) - the formation of typical dermal bony structure with apical and basal compact bony layers and medial trabecular one, all lying in the hypoderma; the second stage (neomorphic) - the appearance of bony callosities as a result of the growth of apical bony layer into the fibrous and papillary ones. The tuberculate sculpture of the surface of bony shell is conditioned by uneven growth of bony tissue around the numerous subepidermal blood vessels.