Studies of the origin and radiation of the molluscs have yet to resolve many issues regarding their nearest relatives, phylogeny and ancestral characters(1-11). The Polyplacophora (chitons) and the Aplacophora are widely interpreted as the most primitive extant molluscs(2,3,9,10), but Lower Palaeozoic fossils of the former lack soft parts, and the latter were hitherto unrecognized as fossils. The Herefordshire Lagerstatte(12) is a Silurian (about 425 Myr BP) deposit that preserves a marine biota in remarkable three-dimensional detail. The external surface of even non-biomineralized cuticle was preserved by entombment in volcanic ash, subsequent incorporation into concretions, and infilling of the fossils with sparry calcite(13). Here we describe, from this deposit, a complete vermiform mollusc, which we interpret as a plated aplacophoran. Serial grinding at intervals of tens of micrometres, combined with computer-based reconstruction methods, renders the fossils in the round(14).