This is a revised and updated edition of Ray Society Monograph Volume 144, originally published in 1962. The contents of this new edition have been modified by the authors in several ways. (Dr Fretter was able to have worked on the revision up to and including the galley-proof stage of production before her death). Certain parts of the original work considered by the authors to be already excellently dealt with in other recent publications or peripheral to the main theme of British Prosobranch Molluscs have been deleted and not replaced. Other parts have been rewritten because new facts and ideas have made the original accounts inadequate, out of date or wrong. The revised edition is now in two parts. Part I contains most of the original text, with some deletions (molluscan radiation outside the gastropods; the physiology and biochemistry of shell secretion; prosobranch parasites; the appendices on classification, habitats and distribution). Part II contains the new chapters. Other parts of the original text have been either extended or replaced and appear in Part II (toxoglossan alimentary anatomy and feeding, the endocrine, excretory and vascular systems, most of the ecology and prosobranch classification and relationships).