This is the first of three volumes covering the marine patellogastropod, vetigastropod and caenogastropod molluscs. This volume is primarily concerned with the two most recognisable (and possibly primitive) subclasses – Patellogastropoda and Vetigastropoda – true limpets and topshells. The keys are based primarily on the characters shown by fully-grown shells (enabling identification of empty shells), but some external body features are included when they offer important aids to identification. Each species account starts with its most useful diagnostic characters, followed by a description of the shell and body. Information on habitat, food, breeding and the general distribution of the species is also provided.