Nature displays several solutions to underwater adhesion. In the bivalve mussels, an adhesive attachment organ known as the byssus is often used. In the blue mussel and related species the byssal system consists of a series of byssus threads that are organic in nature. In contrast hereto, the jingle shell, Anomia sp., has a single large mineralized byssus that extends through the bottom shell. Here we investigate the architecture and composition of the Anomia simplex byssus using scanning electron microscopy and energy dispersive X-ray spectroscopy. We show that the byssus is organized into a hierarchical assembly of crystals and organic matrix. There is a distinct magnesium distribution that is likely to reflect a combination of polymorph and chemical composition control. Sulfur is found to be distributed in distinct zones and sulfur-containing organic matrix provides interconnections between soft tissue and the mineralized byssus. Powder X-ray diffraction shows that calcite and aragonite are present in roughly equal ratios: 55.5(5) wt% aragonite by Rietveld refinement.