Machaeridians were elongate, slug- or worm-like organisms with an external skeleton of imbricating calcite plates (sclerites). Known only from Palaeozoic marine environments, three groups are recognized—plumulitids, turrilepadids and lepidocoleids—but their zoological affinities are enigmatic. Withers (1926) recognized that they formed a monophyletic group, but was uncertain of their systematic position. He did note similarities between the optical properties of machaeridian sclerites and echinoderm ossicles, leading many later workers to interpret machaeridians as aberrant members of the Echinodermata. However, Bengtson (1978) showed this to be erroneous, and subsequent studies have suggested variously that machaeridians are most closely related to arthropods, annelids or molluscs. Using specimens from the Much Wenlock Limestone Formation (Silurian) of England, new information is presented here that indicates machaeridians are molluscs, probably the sister-group of Polyplacophora + Conchifera.