The creation of structural composites with combined strength, toughness, low density, and biocompatibility remains a long-standing challenge. On the other hand, bivalve marine shells – Clinocardium exhibit strength, stiffness and toughness that surpass even that of the nacre which is the most widely mimicked model for structural composites. The superior mechanical properties of Clinocardium shells originate from their cross-lamella design, comprising CaCO3 mineral platelets arranged in an ‘interlocked’ herringbone fashion. Reproduction of such hierarchical designs could offer multifunctionality, potentially combing strength and toughness at low densities and capability for seamless integration with biological systems. Here, we demonstrate manufacturing of the cross-lamella design by biomineralizing aragonite films with saw-tooth patterns and assembling them in a chitosan/fibroin matrix to generate a composite with interlocked mineral layers. The resultant composite —with a similar composition to the biological counterpart, nearly doubles the strength of previous nacre-mimetic composites while improving the tensile toughness and simultaneously exhibiting stiffness and biocompatibility.